Oocytes obtained from superstimulated rhesus monkeys are needed for in vitro fertilization experiments. The animals become refractory to repeated ovarian stimulation by pregnant mare serum gonadotropin (PMSG). Production of antibodies against PMSG was suspected. Antibodies were not detected by gel-diffusion tests, but were found using ELISA procedures in sera from all monkeys that had been treated with PMSG. Sera from seven of nine treated monkeys had antibodies that cross-reacted with FSH-P. An ovarian stimulation test using golden hamsters showed that sera from PMSG-treated rhesus monkeys completely blocked superstimulation of follicle development and ovarian weight gain by PMSG. This study demonstrates that the refractoriness of rhesus monkeys to repeated ovarian stimulation by PMSG is due to the production of nonprecipitating antibodies to PMSG that effectively block the activity of PMSG on the ovary. Formation of these antibodies may preclude use of other gonadotropin preparations in refractory monkeys. Alternative ovarian stimulation procedures avoiding use of gonadotropin preparations need to be developed to permit repeated treatment of rhesus monkeys for multiple oocyte collection.