Abstract: A protein which satisfies all the biological criteria which are characteristic of inhibin has been isolated from a gonadal source. The purification and characterization of inhibin and the use of the purified material to raise antibodies, the use of inhibin and said antisera in a quantative radioimmunoassay, and application in vitro and in vivo of inhibin and antibody to inhibin, are described. There is provided a purified protein, inhibin characterized in that the apparent molecular weight as determined by SDS-PAGE is 56,000.+-.1,000, the isoelectric point is in the range 6.9-7.3, and the protein can bind specifically to Concanavalin A-Sepharose. Moreover, the protein includes two subunits, characterized in that their apparent molecular weights as determined by SDS-PAGE are 44,000.+-.3,000 and 14,000.+-.2,000, respectively. Furthermore, the isoelectric point of the 44,000 molecular weight sub-unit is in the range of 6.0-7.0. In addition, the N-terminal amino acid sequence of the two subunits are described.