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Of 9 hamsters immunized with diphtheria toxin-hamster gamma-globulin conjugates and rechallenged with this antigen 15 days later, 8 exhibited neutralizing antibodies against the conjugates 22 days after the initial immunization. Hamsters identically immunized, but also given cyclophosphamide (CY) 20 hours after the first immunization, had a much lower incidence of positive responses (2/10) and lower antibody levels in the positive responders. After further immunization with the conjugates, all CY-treated hamsters eventually developed neutralizing antibodies, but exhibited a reduction in antibody levels after a second exposure to CY. Hamsters immunized with diphtheria toxoid developed antibodies that were effective in neutralizing diphtheria toxin, but less effective against the diphtheria toxin conjugates.