Methods are disclosed for regulating ovulation or fertility in female mammals, for regulating spermatogenesis in males and for treating conditions such as endometriosis. Administration of effective amounts of an FSH-Inhibiting Protein (FSH-IP) can be used for female contraception and also for male contraception by preventing sperm production. FSH-IP, in its native form, is a glycosylated protein having an apparent molecular weight of about 50,000 Daltons (50kD) which inhibits the production of estradiol that would otherwise be stimulated by FSH in certain cells, such as granulosa cells. Antibodies to these FSH-IP proteins, preferably of monoclonal form can be produced using techniques presently known in the art and are useful for treatment to promote ovulation or superovulation in mammals, including humans and livestock.