Immunization against the $\beta $ subunit of HCG terminates early pregnancy in marmoset and in baboons. Active immunization suppresses fertility in marmosets for about a year if no boosters are given, but when the titres of circulating antibodies drop there is an extended period when abortions recur progressively later in pregnancy. Passive immunization during early pregnancy in marmosets and bonnet monkeys has no marked effect on subsequent fertility. Attempts were made to reverse the effects of passive immunization, using implants of progesterone, with limited success. Our results suggest that the antibodies act to block the luteotrophic support of the corpus luteum and may also affect the placenta or the developing foetus. Attempts to develop synthetic polypeptide fractions of the $\beta $ subunit molecule should reduce the risks of cross-reactions with endogenous hormones. The conjugation of purified fractions of the $\beta $ subunit to non-toxic adjuvants should make immunization of humans a realistic proposal in the future.