The effect of preimmunization with carrier protein on the subsequent response to immunization with hapten-protein conjugate was investigated. Rabbits immunized against bovine serum albumin (BSA) showed a delay in the production of anti-progesterone antibodies upon immunization with progesterone-11-BSA. After a booster injection with P-11-BSA, however, they achieved antiprogesterone levels comparable to those in animals immunized with P-11-BSA only. Similarly, rhesus monkeys preimmunized with tetanus toxoid (TT) showed a delay in the development of anti-hCG titers when immunized with beta-hCG-TT. Here again, after booster immunizations they achieved anti-hCG levels comparable to those of the control animals.