Ovaries from rats treated with pregnant mare serum gonadotrophin were used as a source of inhibin for raising monoclonal antibodies. The antisera were screened for binding to rat ovarian tissue sections and examined for their ability to neutralize inhibin bioactivity in vitro, i.e. the ability to abolish the suppressive action of rat inhibin on pituitary FSH secretion. Two monoclonal antibodies to rat ovarian inhibin were raised, one exhibiting complete (antibody PHM15) and the other incomplete (antibody MCA-3) neutralizing ability. Both antisera bound well to antral granulosa cells, as judged by the degree of staining using an indirect horseradish peroxidase technique. Ascites fluid from mice injected with hybridoma cells secreting antibody PHM15 also showed inhibin-neutralizing ability, and was effective against inhibin prepared from ovarian follicular fluid of rats, sheep, pigs and cows. The latter observation indicates the potential application of this antibody to the preparation and measurement of inhibin from several animal species.