United States securities and exchange commission logo August 12, 2021 Sean A. McCarthy, D.Phil. President and Chief Executive Officer CytomX Therapeutics, Inc. 151 Oyster Point Blvd., Suite 400 South San Francisco, CA 94080 Re: CytomX Therapeutics, Inc. Registration Statement on Form S-3 Filed August 5, 2021 File No. 333-258510 Dear Dr. McCarthy: This is to advise you that we have not reviewed and will not review your registration statement. Please refer to Rules 460 and 461 regarding requests for acceleration. We remind you that the company and its management are responsible for the accuracy and adequacy of their disclosures, notwithstanding any review, comments, action or absence of action by the staff. Please contact Deanna Virginio at 202-551-4530 with any questions. Sincerely, Division of Corporation Finance Office of Life Sciences cc: Mark V. Roeder, Esq.