Summary: The antigonadotropic effect and precipitin titer of the blood of monkeys injected with relatively small amounts of pregnant mare serum having gonadotropic activity, or a highly purified gonadotropic hormone prepared from pregnant mare serum, or the serum of the non-pregnant mare were determined. These substances were given in a series of injections at intervals of 30 days or more and during a period of several months. Monkeys which received pregnant mare serum showed relatively high precipitative titers and antigonadotropic action 10 days after the last injection of the third and subsequent series of injections, but decreased markedly 30 days or more after the last injection of each series. Reinjections restored both substances. The monkeys injected with non-pregnant mare serum produced precipitins but no antigonadotropic substances, whereas the blood of monkeys given the purified gonadotropic extract showed an anti-substance but no evidence of precipitins. The results show that the antigonadotropic substance can be produced by the injection of relatively small quantities of hormone and also indicate that precipitins are not responsible for the antigonadotropic action of the sera. The similarity in the time of appearance and disappearance of the anti-substance and precipitins offers additional evidence for the concept that the anti-substance may be an antibody.