Two 32,000-dalton proteins with inhibin activity were isolated from porcine follicular fluid using heparin-Sepharose affinity chromatography, followed by gel filtration on Sephacryl S-200 and then four steps of reverse-phase high-performance liquid chromatography. Each isolated molecular is composed of two chains having molecular weights of about 18,000 and about 14,000 daltons, respectively, which are bound together by disulfide bonding. Microsequencing revealed the NH.sub.2 -terminal portion of the 18K chain of both to be Ser-Thr-Ala-Pro-Leu-Pro-Trp-Pro-Trp-Ser-Pro-Ala-Ala-Leu-Arg-Leu-Leu-Gln-Arg-Pro-Pro-Glu-Glu-Pro-Ala-Val, of one of the 14K chains to be Gly-Leu-Glu-Cys-Asp-Gly-Arg-Thr-Asn-Leu-Cys-Cys-Arg-Gln-Gln-Phe-Phe-Ile-Asp-Phe-Arg-Leu-Ile-Gly-Trp, and of the other 14K chain to be Gly-Leu-Glu-Cys-Asp-Gly-Lys-Val-Asn-Ile-Cys-Cys-Lys-Lys-Gln-Phe-Phe-Val-Ser-Phe-Lys-Asp-Ile-Gly-Trp-Asn-Asp-Trp-Ile-Ile-Ala-Pro. Both proteins have now been completely characterized, each having a first chain 134 residues long linked by disulfide bonding to a second chain 116 or 115 residues long. The first chain is believed to be glycosylated, which accounts for the disparity between the number of residues and the apparent molecular weight of 18K. These 32K proteins specifically inhibit basal secretion of FSH, but not of LH, in a rat anterior pituitary monolayer culture system. The half-maximal effective does of one is 450 pg/ml and of the other is 900 pg/ml.