Diphtheria toxoid concentrates, final products containing diphtheria toxoid and media used for the production of diphtheria toxin were tested for the presence of bovine sensitizing antigens by active anaphylaxis and passive cutaneous anaphylaxis in guinea pigs. Bovine antigens were detected in toxoids prepared by two manufacturers. The presence of these antigens was related to the type of medium used for toxin production. Bovine sensitizing antigens were not detected in concentrates of tetanus toxoid. These data extend those of Stainer (1967) and indicate that tests to detect the presence of extraneous sensitizing substances should be included in the control of toxoids and vaccines.