Abstract
Anti-sera were produced in rabbits by immunization with purified antifertilizin of Lytechinus and of Urechis and were photo-oxidized so as to convert the antibodies into the "univalent" (non-agglutinating) form. Sperm treated with such anti-sera were found to possess less than 1/64 of the fertilizing capacity of the controls although their activity was not noticeably affected. The bearing of the results on the significance of antifertilizin in the fertilization reaction is briefly discussed and the possible general use of "univalent" antibodies is mentioned.