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                                                        EXHIBIT 10.12


                    [LETTERHEAD OF ORAVAX, INC. APPEARS HERE]





                                          February 10, 1997




Mr. C. Boyd Clark
President and COO
U.S. Bioscience
One Tower Bridge
100 Front Street, Suite 400
West Conshohocken, PA 19248

Via Facsimile:  610-832-4500

Dear Boyd:

I have very much enjoyed our brief discussions regarding the possibility of your
joining OraVax's Board of Directors and look forward to meeting you in person
tomorrow. I hope the general information you've already received from my office
gives you some insight into the position and direction of OraVax. I look forward
to giving you a more subjective view of life at OraVax and answering any
questions you may have. I prepared this letter as background for our
discussions.

The following is a brief summary of some aspects of OraVax Board of Directors
membership:

-     The Board of Directors meets quarterly on a schedule set by the members in
      advance of each year.

-     Board meetings are typically held from 9:00 am until 12:30 pm at the
      Company's facilities in Cambridge, Massachusetts.

-     Board members receive read-ahead material consisting of an agenda, minutes
      from the previous meeting, financial reports, a review of Company
      activities and any issues to be covered at the Board meetings one week
      prior to each meeting.

-     If scheduling difficulties should arise which necessitate the meetings
      being held four or more months apart, conference calls are scheduled
      roughly mid-period to keep the Board current on our operations.
      Abbreviated pre-mail packages are sent out in advance of such scheduled
      conference calls.

-     I also send out a brief President's letter to the Board members during
      those months where more thorough pre-meeting material is not distributed.
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-     Mr. MacMaster, the Chairman of our Board, spends a full day at the Company
      on the day preceding Board meetings and typically at least one additional
      day between Board meetings.

-     Informal telephone discussions between myself and one or more Board
      members occur on an as needed basis, but I don't believe they are viewed
      by anyone as a burden.

-     The Board has two active subcommittees, a Compensation Committee and an
      Audit Committee. You would likely be asked to sit on one or the other of
      these committees. The committees meet approximately twice yearly and are
      typically held immediately preceding or following general Board meetings.

-     We have customary Directors and Officers insurance and are a Delaware
      corporation with the customary provisions limiting Directors and Officers
      liability.

-     Current compensation for Board members includes the following:

            -     A $5,000 annual retainer

            -     $1,500 per Board meeting attended

            -     Options for 10,000 shares of OraVax stock to vest over a three
                  year period

            -     OraVax reimburses its Board Members for any travel expenses
                  and reasonable out of pocket expenses associated with Board
                  membership

I hope the above meets your expectations. I look forward to meeting with you.

Sincerely,

/s/ Lance K. Gordon

Lance K. Gordon
President & CEO
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Summary of Directors & Officers Insurance


Total coverage:         $5,000,000

Two layers:             Primary $2,000,000; Lloyds of London
                        Excess $3,000,000 American Dynasty (AM Best A,XI)

Allocation:             100% predetermined allocation for SEC claims

Indemnity:              Bylaws provide customary indemnity to officers