Mullerian inhibiting substance (MIS), a glycoprotein responsible for the regression of Mullerian duct in the male mammalian embryo, was recently localized not only in the fetal and newborn testis, but also in the older ovary throughout reproductive life. Bovine MIS purified from newborn testicular tissue inhibited spontaneous oocyte meiosis in vitro in the rat. These studies show that partially purified recombinant MIS produced from a human MIS genomic construct caused inhibition of oocyte meiosis, but when purified to homogeneity, the effect was lost. Addition of low concentrations of the detergent Nonidet P-40, used to maintain stability in the purified bovine preparations, however, restored the MIS inhibitory effect to the human preparation, which could, in turn, be blocked by a polyclonal antibody raised to human recombinant MIS; Nonidet P-40 itself caused no inhibition. Since we have shown that epidermal growth factor (EGF) and MIS are antagonistic in a number of other systems, we tested the effect of EGF on the ability of MIS to inhibit oocyte meiosis. EGF added to the medium at a dose that caused no effect on oocytes (25 ng/ml) blocked the MIS inhibitory action on spontaneous rat oocyte meiosis, while EGF had no effect on a known oocyte meiosis inhibitor, 3-isobutyl-1-methylxanthine. These data indicate that human recombinant MIS can inhibit oocyte meiosis in the ovary and that its regulatory effect can be modulated by EGF, which appears to be an antagonist of MIS.