SecProbe.io

Mail Stop 3561

September 4, 2015

Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
343 Oyster Point Blvd., Suite 100
South San Francisco, CA 94080

Re: CytomX Therapeutics, Inc.
Registration Statement on Form S -1
Filed August 28, 2015
  File No. 333 -206658

Dear Mr.  McCarthy :

We have reviewed your amended registration statement and have the following
comments.  In some of our comments, we may ask you to provide us with information so we
may better understand your disclosure.

Please respond to this letter by amending your registration statement and providing the
requested information.  If you do not believe our comm ents apply to your facts and
circumstances or do not believe an amendment is appropriate, please tell us why in your
response.

After reviewing any amendment to your registration statement and the information you
provide in response to these comments, we m ay have additional comments.   Unless we note
otherwise, our references to prior comments are to comments in our August 18, 2015 letter.

The Offering, page 8

1. Please update the common stock outstanding after the offering calculation based on the
most recent practicable date.

Use of Proceeds, page 51

2. We reissue prior comment 3.  We continue to note the broad discretion to change the use
of proceeds in the future as discussed in this section and the risk factor on page 44.
Please revise to clearly add ress the contingencies discussed with specificity and the
alternatives.  See Instruction 7 to Item 504 of Regulation S -K.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 4, 2015
Page 2

Employees, page 117

3. We reissue prior comment 7.  Your disclosure should address all employees, not just full
time employees.  Pleas e revise.  See Item 101(c)(xiii) of Regulation S -K.

Executive Compensation , page 127

4. We reissue prior comment 10.  We note the disclosure in footnote one.  The narrative
following the table indicates that the grant date fair value equaled the cash retainer fees.
However, the dollar amounts disclosed in the narrative following the table are not the
same value as that included in the table.  For example, you stat e Dr. Huh’s annual
retainer was $80,000, but the amount included in the table is $160,000.  Please reconcile
or advise.

5. We reissue comment 11.  Please disclose the specific items of corporate performance that
are taken into account in the formula or cri teria in determining the non -equity incentive
award.  See Item 402(o)(5) of Regulation S -K.

Exhibits

6. Please file Exhibits 10.15 and 10.21 in their entirety including the exhibits, schedules
and/or attachments.

We urge all persons who are responsible for the accuracy and adequacy of the disclosure
in the filing to be certain that the filing includes the information the Securities Act of 1933 and
all applicable Securities Act rules require.  Since the company and its management are in
possession of all facts relating to a company’s disclosure, they are responsible for the accuracy
and adequacy of the disclosures they have made.

Notwithstanding our comments, in the event you request acceleration of the effectiv e date
of the pending registration statement please provide a written statement from the company
acknowledging that:

should the Commission or the staff, acting pursuant to delegated authority, declare the
filing effective, it does not foreclose the Com mission from taking any action with respect
to the filing;

the action of the Commission or the staff, acting pursuant to delegated authority, in
declaring the filing effective, does not relieve the company from its full responsibility for
the adequacy a nd accuracy of the disclosure in the filing; and

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 4, 2015
Page 3

 the company may not assert staff comments and the declaration of effectiveness as a
defense in any proceeding initiated by the Commission or any person under the federal
securities laws of the United States.

Please refer to Rules 460 and 461 regarding requests for acceleration. We will consider a
written request for acceleration of the effective date of the registration statement as confirmation
of the fact that those requesting acceleration are aware  of their respective responsibilities under
the Securities Act of 1933 and the Securities Exchange Act of 1934 as they relate to the proposed
public offering of the securities specified in the above registration statement.  Please allow
adequate time for u s to review any amendment prior to the requested effective date of the
registration statement.

Please contact Ruairi Regan at (202) 551 -3269 or Pamela Howell, Special Counsel, at
(202) 551 -3357 with any questions.

     Sincerely,

     /s/ Pamela Howell
     for

     John Reynol ds
Assistant Director
Office of Beverages,
Apparel, and Mining

cc:  Sam Zucker, Esq.
       Sidley Austin LLP