UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                                  SCHEDULE 13D
                   Under the Securities Exchange Act of 1934
                               (Amendment No. 4)*


                                  AEROGEN, INC.
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                                (Name of Issuer)

                     Common Stock, par value $0.001 per share
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                         (Title of Class of Securities)

                                    007779309
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                                 (CUSIP Number)

   Mitchell D. Kaye, Manager                        with a copy to:
   Xmark Asset Management, LLC                      Steven E. Siesser, Esq.
   301 Tresser Blvd.                                Lowenstein Sandler PC
   Suite 1320                                       65 Livingston Avenue
   Stamford, Connecticut 06901                      Roseland, New Jersey  07068
   (203) 653-2511                                   (973) 597-2506
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                 (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                  April 4, 2005
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             (Date of Event which Requires Filing of this Statement)


If the filing person has previously  filed a statement on Schedule l3G to report
the  acquisition  that is the subject of this  Schedule  13D, and is filing this
schedule  because of Sections 240.13d-1(e),  240.13d-1(f) or 240.13d-1(g), check
the following box. [ ]

Note:  Schedules  filed in paper format shall include a signed original and five
copies of the schedule, including all exhibits.  See Section 240.13d-7 for other
parties to whom copies are to be sent.

* The remainder of this cover page shall be filled out for a reporting  person's
initial filing on this form with respect to the subject class of securities, and
for  any  subsequent   amendment   containing   information  which  would  alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the  Securities  Exchange  Act of
1934 ("Act") or otherwise  subject to the liabilities of that section of the Act
but  shall be  subject  to all other  provisions  of the Act  (however,  see the
Notes).




Cusip No.         007779309
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1.  Names of Reporting Persons.  I.R.S. Identification Nos. of above persons
   (entities only):
                       Xmark Asset Management, LLC
                               13-3954392
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2.  Check the Appropriate Box if a Member of a Group (See Instructions):
         (a)                  Not
         (b)               Applicable
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3.  SEC Use Only
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4.  Source of Funds (See Instructions):  WC
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5.  Check if  Disclosure  of  Legal  Proceedings Is  Required Pursuant  to Items
    2(d) or 2(e):                 Not Applicable
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6.  Citizenship or Place of Organization:  New York, United States
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    Number of                      7. Sole Voting Power:                  *
                                      ------------------------------------------
    Shares Beneficially            8. Shared Voting Power:                *
                                      ------------------------------------------
    Owned by
    Each Reporting                 9. Sole Dispositive Power:             *
                                      ------------------------------------------
    Person With                   10. Shared Dispositive Power:           *
                                      ------------------------------------------
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11. Aggregate Amount Beneficially Owned by Each Reporting Person:   1,133,330*
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12. Check if the Aggregate Amount in Row (11) Excludes Certain Shares
    (See Instructions):          Not Applicable
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13. Percent of Class Represented by Amount in Row (11):   18.8%*
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14. Type of Reporting Person (See Instructions):  IA
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*   Xmark Asset Management, LLC, a New York limited  liability  company ("XAM"),
serves as investment  manager for each of Xmark Fund,  L.P., a Delaware  limited
partnership  ("Xmark  LP"),  and Xmark Fund,  Ltd.,  a Cayman  Islands  exempted
company  ("Xmark Ltd").  In such  capacity,  XAM possesses the power to vote and
direct the  disposition of all securities  held by Xmark LP and Xmark Ltd. As of
April  4,  2005,  Xmark  LP is the  holder  of  50,337  shares  (the  "Xmark  LP
Preferred") of the Series A-1 Preferred  Stock,  par value $0.001 per share (the
"Preferred  Stock") of Aerogen,  Inc., a Delaware  corporation  (the "Company"),
which are presently  convertible  into 503,370  shares of the  Company's  common
stock,  par value  $0.001 per share (the "Common  Stock").  As of April 4, 2005,
Xmark  Ltd is the  holder  of 62,996  shares  (the  "Xmark  Ltd  Preferred")  of
Preferred Stock,  which are presently  convertible into 629,960 shares of Common
Stock.  As a result of the fact that XAM  possesses  the sole  power to vote and
direct the disposition of the securities  described  above, for purposes of Reg.
Section  240.13d-3,  XAM may be deemed to beneficially  own 1,133,330  shares of
Common Stock, or approximately 18.8% of the shares of Common Stock deemed issued
and outstanding as of April 4, 2005.





Item 4.   Purpose of Transaction.
          ----------------------

          Item 4 of the  Schedule  13D is hereby  amended  and  restated  in its
entirety as follows:

          As the beneficial owner of the percentage of the Company's outstanding
Common Stock  described in Item 5 of this  Schedule  13D, XAM can  influence the
outcome  of  matters  that  may be  submitted  to  the  vote  of  the  Company's
stockholders  at annual or special  meetings  of  stockholders.  Such  corporate
actions may  include,  and are not limited to, the approval of each of the types
of actions  described in clauses (a) through (g) of Item 4 of Schedule 13D under
the Securities Exchange Act of 1934, or any similar actions.

          XAM also has the power to  influence  the  strategic  direction of the
Company as a result of  restrictive  covenants  contained in the  Certificate of
Designations  of the Series A-1 Preferred  Stock,  par value $0.001 per share of
the Company (the "Preferred Stock") described in Item 6 of this Schedule 13D. In
addition,  under the terms of the Purchase Agreement described in Item 6 of this
Schedule  13D,  Xmark  LP and  Xmark  Ltd  have the  right  (which  has not been
exercised to date) to designate two members to the Company's Board of Directors,
and to remove or replace their designees,  until they no longer collectively own
at least  80,000  shares  of  Preferred  Stock in the  aggregate  (appropriately
adjusted for any stock dividend, split, combination or other recapitalization).

          Since the  completion of the Financing,  XAM has repeatedly  contacted
the  Company  and its Board of  Directors  expressing  strong  concern  with the
Company's  strategic  direction.  In  particular,  XAM has  expressed  that  the
Company's greater than expected  dissipation of cash since the completion of the
Financing,  combined with management's inability to lead the Company responsibly
and to  properly  administer  its  financial  resources,  has  compelled  XAM to
question  the  continuing  efficacy of current  management.  XAM has  repeatedly
requested  that  the  Company's  management  and  Board  implement  cost-cutting
measures  and retain an  investment  banker to explore the  Company's  strategic
alternatives.   In  response,   the  Company   rebuffed  XAM's  concerns  as  an
unintelligent  misunderstanding  of the Company's  business model, while stating
that it was indeed responsibly managing the Company.

          On March 29, 2005,  XAM sent a letter to the Company's  Board to, once
again,  express its deepening  concern with the  increasing  degradation  of the
Company's  financial  condition and its continued terrible  performance,  and to
provide  the  Company  with  notice  that it will  not  consent  to any  Company
financing  transaction  presented  to it  unless  the  Company  effects  (i)  an
immediate  change in management,  including the  replacement of the Chairman and
Chief Executive Officer and the Chief Financial  Officer of the Company,  (ii) a
sale of all  non-therapeutic  assets  (e.g.,  the  device  business),  (iii) the
immediate  implementation  of a  plan  to  reduce  expenditures,  and  (iv)  the
retention  of a  qualified  investment  banking  firm to assist  the  Company in
exploring alternatives to maximize shareholder value, including a potential sale
of the  Company.  This letter is attached  as an Exhibit to  Amendment  No. 3 to
Schedule 13D pursuant to Item 7 thereof,  and the  description of this letter is
qualified in its entirety by this reference thereto.





          On April 1, 2005,  Dr. Jane Shaw,  on behalf of the  Company's  Board,
responded to XAM's March 29, 2005 letter.  Dr. Shaw's letter is filed as Exhibit
99.1 to the Current Report on Form 8-K filed by the Company on April 1, 2005. In
her  response,  Dr. Shaw  asserts,  among other  things,  that XAM lacks certain
information  about the  Company  and XAM  continues  to fail to  understand  the
Company.  Dr. Shaw's letter goes on to state that the Company's  Board has acted
responsible  since  XAM's 2004  investment  in the Company and in what the Board
believed to  be the  stockholder'  best  interest.  Through Dr. Shaw,  the Board
reiterated its offer to XAM to exercise its rights to appoint two members to the
Board.  The description of this letter is qualified in its entirety by reference
thereto.

          On April 4, 2005,  XAM  responded  to Dr.  Shaw's April 1, 2005 letter
disputing what it viewed as mischaracterizations  and misstatements contained in
her letter.  XAM asserted that Dr. Shaw was attempting to shift the focus of the
debate from Dr. Shaw's and the Board's conduct and the Company's  performance to
XAM. In its response letter, XAM reiterated its desire to support the removal of
the Company's  current  management team, and also stated it would consider using
its voting power to replace the Board.  This letter is attached as an Exhibit to
this  Amendment  No. 4 to  Schedule  13D  pursuant  to Item 7  thereof,  and the
description  of this  letter is  qualified  in its  entirety  by this  reference
thereto.

          XAM may,  from time to time,  further  utilize  its  voting  and other
rights to change or influence control of, and to influence the corporate affairs
of, the Company,  including  through the use of its influence on the approval of
any of the events referred to above.  XAM may take additional  steps intended to
replace  management,  including without limitation Dr. Jane Shaw, and/or certain
or all of the  members  of the  Board  of  Directors.  XAM may  seek to  acquire
additional  shares of the  Company's  Common Stock in the open market or through
private  purchase  transactions,  in an effort to increase  its position and its
voting  strength.  XAM also may decide to dispose of all or part of its position
if it  determines  that it no longer  desires to be, or it desires to reduce its
position as, a stockholder of the Company.

          XAM  intends  to  continue  to  evaluate  the  business  and  business
prospects of the Company and its present and future  interest in, and intentions
with respect to, the Company,  and in connection therewith may from time to time
consult with other stockholders of the Company.

          The descriptions of the documents set forth in this Amendment No. 3 to
Schedule  13D are  qualified  in their  entirety by  reference  to the  complete
agreements  governing such matters,  each of which are incorporated by reference
to this Amendment No. 3 to Schedule 13D as exhibits pursuant to Item 7 hereof.


Item 7.   Material to be Filed as Exhibits.
          --------------------------------

          Item 7 of the Schedule 13D is hereby amended by adding the following:

          11.  Letter dated  April 4, 2005 from  XAM to the  Company's Board  of
Directors.





                                    Signature
                                    ---------

          After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information  set forth in this statement is true,  complete and
correct.


                                            April 4, 2005

                                            XMARK ASSET MANAGEMENT, LLC


                                            /s/ Mitchell D. Kaye
                                            ------------------------------------
                                            Mitchell D. Kaye, Manager


Attention:  Intentional  misstatements  or omissions of fact constitute  Federal
criminal violations (See 18 U.S.C. 1001).






                                                                      Exhibit 11



April 4, 2005


Board of Directors
Aerogen, Inc.
2071 Stierlin Court
Mountain View, California  94043


Ladies and Gentlemen of the Board:

This letter responds to the mischaracterizations and misstatements in Dr. Shaw's
April 1, 2005  letter  to Xmark.  The  focus in that  letter is  misplaced.  The
company's focus should not be on Xmark's failure to continue  accepting material
non-public information from the company, which Xmark is not obligated to accept.
Nor  should the focus be  Xmark's  refusal to serve on a feeble and  subservient
board of directors, thereby exposing itself to personal liability from a suit by
the company's  shareholders.  The focus, instead, should solely be on Dr. Shaw's
own ineptitude in leading this company and the Board's corresponding impotence.

In her letter,  Dr. Shaw stated that we do not understand the company's business
model.  We will never  understand a model in which the chief  executive  officer
wantonly and repeatedly drives her company to the brink of bankruptcy.  We have,
despite her erroneous and  misleading  statements to the contrary,  had multiple
meetings with management. We also have had countless telephonic discussions with
management and the Board of Directors.

The only true lack of understanding  here lies with the company.  Management and
the Board clearly do not understand the company's investors, particularly Xmark,
which  led last year's  financing  that rescued the company.  Despite Dr. Shaw's
assertion,  we never  expected a quick exit from our  investment in the company.
Xmark is a long-only life sciences fund and our investment thesis was based on a
long-term perspective.

Dr. Shaw also  complained that our filings made with the Securities and Exchange
Commission  were late.  That  complaint is audacious  given that the company has
missed  the filing  deadline  for its  Annual  Report on Form  10-K.  Dr. Shaw's
complaint  is even more  egregious  given that the company has never once timely
paid the dividends on the preferred  stock.  Indeed,  the company has missed the
timely payment every single  quarter since the preferred  stock has been issued.
In most cases,  the company has been more than four months late in paying  these
dividends.  The Board  certainly  must  recognize that this failure to pay these
dividends renders the company ineligible to use a Form S-3 for further financing
transactions.  This  significantly  increases  the cost and time it will take to
complete  any  such  financing.  This  is just  one  more  manifestation  of the





Board of Directors of Aerogen, Inc.
April 4, 2005
Page 2 of 3



pervasiveness of the company's and the Board's mismanagement and disregard.  The
delinquency of these dividends is laid equally at  management's  and the Board's
doorsteps.

Our  recently  reported  securities  transactions,  nearly  one year  after  our
investment,  were  accomplished not only with the Board's and management's  full
knowledge, but indeed, in some cases, with their assistance and approval. As you
well  know,  and as you were  timely  notified,  we sold a small  portion of our
preferred position in a private sale. We also sold our unexercised warrants in a
private  sale.  Finally,  we sold 91,309  dividend  shares of common  stock in a
private  sale.  None of these  trades were in or through  the  market.  In fact,
during the period  covered by these  filings,  we have never been in the market,
long or short, in the company's stock.

Dr. Shaw's mischaracterizations and misstatements in her letter are nothing more
than a  transparent  attempt to shift focus away from her  incompetence  and the
company's abhorrent performance.

We want to remind  management  and the Board as to the  purpose of our letter of
last  week.  As  long  as  current  management  remains  in  place  -and  absent
significant and immediate changes in circumstances- we will absolutely  exercise
our veto right with  respect to any  financing  transaction  over which our veto
would have effect. Dr. Shaw has time and again proven her gross incompetence and
displayed  wanton  disregard for anything but her own reputation.  We frankly do
not  understand  how  someone  who  serves on the boards of  directors  of Intel
Corporation, McKesson Corporation and OfficeMax Incorporated - three Fortune 500
companies - is so incompetent at running a small public company.

It seems apparent to us that Dr. Shaw has a stranglehold on the Board, which has
been paralyzed by Dr. Shaw's  reputation.  The Board, in our view, has subverted
its fiduciary duties in favor of Dr. Shaw's misguided agenda.  The Board appears
unable to independently evaluate its responsibilities and obligations.  A number
of months ago we suggested to the Board that we believed  company  counsel had a
conflict of interest in  advising  both  management  and the Board (of which Dr.
Shaw is  Chairman)  throughout  this  ongoing  debate.  The  Board's  continuing
reliance on company  counsel in this matter  demonstrates to us that it is in no
way  independent  from Dr. Shaw. In our view,  this gross lapse in judgment is a
very material issue.

If we have not been clear enough already, we will reiterate our point once again
- we are  nearing the end of our  patience.  The waste of  corporate  assets and
incompetent  administration of the company is, in our view,  actionable.  If the
circumstances do not change  immediately,  including Dr. Shaw's removal, we will





Board of Directors of Aerogen, Inc.
April 4, 2005
Page 3 of 3


explore taking  definitive steps to enforce our rights and remedies against both
management and the Board.


Very truly yours,

Xmark Asset Management, LLC


/s/ Mitchell D. Kaye
---------------------------
Mitchell D. Kaye,
Chief Investment Officer