Rhesus monkeys immunized with the beta-subunit of ovine luteinizing hormone (oLHbeta) developed circulating antibodies which cross-reacted strongly with rhesus monkey chorionic gonadotropin. Normal ovulatory cycles continued, but the fertility of immunized monkeys as compared with that of controls was significantly reduced. Thus, the rhesus monkey represented a useful animal model for the study of certain aspects of an "antifertility vaccine." We investigated the effects of circulating antibodies to oLHbeta on corpus luteum function by measuring production rates (PRs) and peripheral concentrations of progesterone during the luteal phase of the menstrual cycle. Both parameters were significantly lower in immunized animals than in control animals. The length of the menstrual cycle was also significantly reduced. Progesterone PRs were also determined on days 10 and 15 after mating to test the assumption that the antifertility vaccine prevents pregnancy by interfering with "corpus luteum rescue" (the increase in PRs of progesterone usually occurring on day 15 after mating in fertilized animals). PRs increased from days 10 to 15 in pregnant controls, were unchanged in nonpregnant controls, and were significantly lower on day 15 in immunized monkeys as compared with nonpregnant and pregnant controls. These results suggest that "corpus luteum rescue" is suppressed in immunized animals.