SecProbe.io

Mail Stop 3561

        September 18, 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.
Amendment No. 1 to Registration Statement on Form S -1
Filed September 11, 2015
Response dated September 18, 2015
  File No. 333 -206658

Dear Mr.  McCarthy :

We have reviewed your supplemental response dated September 18, 2015  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 comments 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 state ment and the information you
provide in response to these comments, we may have additional comments.

The Offering, page 8

1. You disclose that the number of shares to be outstanding after the offering is based on
28,259,170 shares of common stock outsta nding as of August 31, 2015 which includes
the conversion of all shares of preferred stock and the net exercise of all outstanding
warrants to purchase shares of preferred stock.  Please tell us how you determined the
amount of common shares to be outstand ing in view of the 27,135,453 converted
preferred shares and 65,884 purchase warrants reflected in footnote 1 to the table on page
11.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 18, 2015
Page 2

Dilution, page 58

2. You state that your historical net tangible book value per share is comprised of your total
tangi ble assets less total liabilities, less preferred stock, divided by the number of your
outstanding shares of common stock.  Please specify the assets that you are excluding
from your total tangible assets and provide your calculations that support your his torical,
pro forma and pro forma as adjusted net tangible book value per share as of June 30,
2015.

Financial Statements

14. Stock Option Plans, page F -29

3. Your disclosures indicate that you granted 697,557 stock options in 2014 with a weighted
average  exercise price of $1.449 and an additional 1,290,705 stock options with a
weighted average exercise price of $2.621 during the six months ended June 30, 2015.
Please address  the following points:

 Provide us with further details for stock and options is sued on dates within the last
year including number of shares/options granted, grant dates, and exercise prices.

 Tell us the methods used and the material assumptions involved in determining the
fair value of the company’s shares in relation to the sto ck or options issued leading up
to the offering.

 To the extent estimated IPO price differs significantly from the most recent fair value
determined for your common shares, please tell us the significant factors and events
contributing to the changes in f air value of the underlying stock leading up to the
expected IPO.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 18, 2015
Page 3

 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 Reynolds
Assistant Director
Office of Beverages, Apparel, and
Mining

cc:  Sam Zucker, Esq.
       Sidley Austin LLP