<PAGE>

                                                                    Exhibit 4.33

(PROTHERICS LOGO)

21 July 2005

Mr Bryan Morton
Zeneus Pharma Ltd
Magdalen Centre
Oxford Science Park
Oxford
OX4 4GA

Dear Bryan

               PROTHERICS PLC (THE "COMPANY") AND YOUR APPOINTMENT
                    TO THE BOARD AS A NON-EXECUTIVE DIRECTOR

Following the recommendation of the nomination committee, the board of the
Company (the "Board") is pleased to hear that you have accepted our offer to
join on the Board as a non-executive director with effect from 1 August 2005.
This letter confirms the main terms of your appointment to this office. You
should be aware that your appointment will have to be ratified by the Company's
shareholders at the annual general meeting following your appointment, and is
subject to the Company's articles of association as amended from time to time.
If there is a conflict between the terms of this letter and the articles of
association then the articles shall prevail.

DUTIES

1.   You will be informed how the Board is structured and what authorities are
     delegated to the Chief Executive and his colleagues.

2.   The Board as a whole is collectively responsible for promoting the success
     of the Company by directing and supervising the Company's affairs. The
     Board's role is to:

     (a)  provide entrepreneurial leadership to the Company within a framework
          of prudent and effective controls which enable risk to be assessed and
          managed;

                                                    Protherics PLC
                                                    Fourth Floor
                                                    3 Creed Court
                                                    5 Ludgate Hill
Registered Office                                   London EC4M 7AA
Registered in England No. 2459087                   Tel: 0207 246 9950
Protherics PLC, The Heath Business and              Fax: 0207 329 5924
Technical Park,                                     Web Site: www.protherics.com
Runcorn, Cheshire WA74QF

<PAGE>

                                        2


     (b)  set the Company's strategic aims, ensure that the necessary financial
          and human resources are in place for the Company to meet its
          objectives, and review management performance; and

     (c)  set the Company's values and standards and ensure that its obligations
          to its shareholders and others are understood and met.

3.   In your role as a non-executive director you are required (with the other
     non-executives) to:

     (a)  constructively challenge and contribute to the development of
          strategy;

     (b)  scrutinise the performance of management in meeting agreed goals and
          objectives and monitor the reporting of performance;

     (c)  satisfy yourself that financial information is accurate and that
          financial controls and systems of risk management are robust and
          defensible;

     (d)  be responsible for determining appropriate levels of remuneration of
          executive directors and have a prime role in appointing, and where
          necessary removing, senior management and in succession planning.

4.   You will be required to:

     (a)  exercise relevant powers under the Company's memorandum and articles
          of association;

     (b)  perform your duties faithfully, efficiently and diligently and use all
          reasonable endeavours to promote the interests and reputation of the
          Company;

     (c)  comply with your fiduciary duties;

     (d)  report the wrongdoing (including acts of misconduct, dishonesty,
          breaches of contract, fiduciary duty, company rules or the rules of
          the relevant regulatory bodies) whether committed, contemplated or
          discussed by any other director or member of staff of the Company and
          any group company of which you were aware to the Board immediately,
          irrespective of whether this may involve some degree of self
          incrimination;

     (e)  serve on the nomination, audit and remuneration committees of the
          Board and attend, wherever possible, all meetings of the committees
          (committee meetings may occasionally be scheduled separately from
          Board meetings);

<PAGE>

                                       3


     (f)  attend all general meetings of the Company;

     (g)  attend, wherever possible, all meetings of the Board, which meets on
          an agreed schedule (in 2005, 9 meetings are scheduled). These are
          mostly at the Company's offices at Ludgate Hill, London, with
          occasional meetings at other Company locations;

     (h)  consider all relevant papers in advance of each meeting in order to
          ensure that you can play a full part in the work of the Board and its
          committees;

     (i)  bring independent judgement to bear on issues of strategy, policy,
          resources, performance and standards of conduct;

     (j)  make yourself available (on reasonable notice) to provide ad hoc
          advice to individual directors of the Company;

     (k)  provide guidance and direction in planning, developing and enhancing
          the future strategic direction of the Company;

     (l)  share responsibility with the other directors for the effective
          control of the Company and with the other non-executive directors for
          the supervision of the executive directors; and

     (m)  comply with the Financial Services Authority's Model Code for
          securities transactions by directors of listed companies and with any
          code of conduct relating to securities transactions by directors and
          specified employees issued by the Company from time to time.

5.   Overall, the Company anticipates that you will need to spend an average of
     one day per month fulfilling your duties after the induction phase. This
     will include the board meetings, annual general meetings, annual strategy
     meeting and site visits. In addition you will be expected to spend an
     appropriate period of time preparing for each meeting. By accepting this
     appointment you confirm that you are able to commit sufficient time to the
     role to meet the Company's expectations.

6.   The Company seeks to adhere to the principles in the Cadbury Report on
     Corporate Governance, the Greenbury Report on Directors' Remuneration, the
     Hampel Report on Corporate Governance and the Turnbull Report on Internal
     Control. You will be expected to carry out your duties in accordance with
     the principles set out in these reports, copies of which are available from
     the Company Secretary.

7.   The performance of the Board and its committees, and of individual
     directors, is evaluated annually.

<PAGE>

                                       4


8.   You shall, in pursuance of your duties hereunder, be entitled to request
     such information from the Company, its subsidiary undertakings (as defined
     in section 258 of the Companies Act 1985 as amended from time to time) or
     its or their employees, consultants or professional advisers as may be
     reasonably necessary to enable you to perform your role effectively. The
     Company shall use its reasonable endeavours to provide such information
     promptly.

CONFIDENTIALITY

During the course of your duties you will have access to confidential
information belonging to the Company and its subsidiary undertakings
(including, but not limited to, details of suppliers, customers, margins,
know-how, marketing and other relevant business information). Unauthorised
disclosure of this information could seriously damage the Company. You therefore
undertake not to use or disclose such information save in pursuance of your
duties or in accordance with any statutory obligation or court or similar order.

Your attention is drawn to the rules relating to the disclosure of price
sensitive information. You must not make any statement or do anything which may
be a breach of these rules without prior clearance from the Chairman or Company
Secretary.

OUTSIDE INTERESTS

The agreement of the chairman should be sought before you accept any new outside
interests which might affect the time you are able to devote to this
appointment.

In accordance with the principles set out in the Combined Code you must inform
the Board of any interests which you have, or acquire, which might reasonably be
thought to jeopardise your independence from the Company.

During your appointment you must not take up any office or employment with, or
have any interest in, any firm or company which is or may be in direct or
indirect competition with the Company.

The Board have determined you to be independent, according to the provisions of
the Combined Code.

INSURANCE

During your appointment you will be covered by the Company's directors' and
officers' liability insurance on the terms currently in place for the rest of
the Board. The current indemnity limit is L10 million. A copy of the policy
document is available from the Company Secretary. The Company does not guarantee
to maintain this insurance cover after the termination of your appointment, but
you will continue to be covered by the policy or any replacement on the same
basis as the rest of the Board.

<PAGE>

                                        5


APPOINTMENT

Your appointment will commence on 1 August 2005 and is terminable by three
months' notice from either the Company or yourself. It is envisaged that your
appointment will initially be for a term of three years. The continuation of
your appointment depends upon satisfactory performance and re-election at
forthcoming Annual General Meetings.

Not withstanding the aforementioned notice provisions, the Company may terminate
your appointment with immediate effect if you have:

(a)  committed any serious breach or (after warning in writing) any repeated or
     continued material breach of your obligations to the Company (which include
     an obligation not to breach your fiduciary duties);

(b)  been guilty of any act of dishonesty or serious misconduct or any conduct
     which (in the reasonable opinion of the Board) tends to bring you or the
     Company into disrepute; or

(c)  been declared bankrupt or have made an arrangement or composition with of
     for the benefit of your creditors,

or for any other reason set out in the Company's articles of association.

All appointments and reappointments to the Board are, of course, subject to the
Company's articles of association. If you are not re-elected to your position as
a director of the Company by the shareholders at any time and for any reason
then this appointment shall terminate automatically and with immediate effect.

On termination of the appointment you shall only be entitled to such fees as may
have accrued to the date of termination together with reimbursement in the
normal way of any expenses properly incurred prior to that date.

REMUNERATION

The fee is L25,000 per annum (subject to income tax and statutory
deductions) and is payable monthly in arrears. It may be subject to upward
review at the discretion of the Company after one year. If you undertake any
special duties beyond those specified in this letter you will not receive any
additional payment unless agreed with the Board in advance.

EXPENSES

The Company will reimburse you for any expenses that you may incur properly and
reasonably in performing your duties and which are properly documented. Such
expenses would include reasonable legal fees if circumstances should arise in
which it was necessary for you to seek separate legal advice about the
performance of your

<PAGE>

                                        6


duties. In such a situation, you are required to discuss the issue either with
me or with one of your non-executive colleagues in advance.

INDEPENDENT PROFESSIONAL ADVICE

In some circumstances you may think that you need professional advice in the
furtherance of your duties as a director. It may also be appropriate for you to
seek advice from independent advisers at the Company's expense. A copy of the
Board's agreed procedure under which directors may obtain such independent
advice is available from the Company Secretary. The Company will reimburse the
full cost of any expenditure incurred in accordance with the policy.

DATA PROTECTION

By signing this agreement you consent to the Company holding and processing
information about you which you may provide or which it may acquire during the
course of this agreement, providing such use is in accordance with the Data
Protection Act 1998. In particular you consent to the Company holding and
processing:

(a)  personal data relating to you, for administrative and management purposes;
     and

(b)  "sensitive personal data" relating to you (as defined in the Data
     Protection Act 1998) including, for example:

          (i)  your health records and any medical reports given to or obtained
               by the Company, for monitoring sick leave and taking decisions as
               to your fitness to work;

          (ii) your racial or ethnic origin (in order to monitor compliance with
               the Race Relations Act 1976);

          (iii) any information relating to criminal proceedings in which you
               have been involved for compliance with the Company's legal or
               regulatory requirements, for insurance purposes, and in relation
               to its obligations to third parties.

You also consent to the Company making such information available to:

(a)  its group companies, offices and (if necessary) customers outside the
     European Economic Area in order to further its business interests; and

(b)  its group companies, its offices, those who provide products or services to
     the Company, regulatory authorities, governmental or quasi governmental
     organisations and potential purchasers of the Company or its business.

<PAGE>

                                        7

THIRD PARTY RIGHTS

The Contracts (Rights of Third Parties) Act 1999 shall not apply to this
agreement. No person other than the parties to this agreement and any group
company shall have any rights under it and it will not be enforceable by any
person other than those parties.

Please sign and return the enclosed copy of this letter to confirm your
agreement to the above terms. We will separately also ask you to sign a form 288
for filing at Companies House.

The Company looks forward to working with you in the future.

Yours sincerely


/s/ Stuart Wallis
-------------------------------------
Stuart Wallis
for and on behalf of Protherics Plc

I, Bryan Morton, agree to the above terms of appointment as a non-executive
director of Protherics Plc.

Name /s/ Bryan Morton
Date 21 July 2005

In the presence of:


Witness signature:  /s/ A. Kutkus
                    -------------------------
Witness name:       ANNA KUTKUS
Witness address:    10 WALDRON MEWS
                    LONDON SW3 5BT
Witness occupation: DESIGNER