Inhibin, a gonadal protein that preferentially suppresses the secretion of pituitary follicle-stimulating hormone, has been isolated from porcine follicular fluid and characterized as a 32-kDa protein composed of 18-kDa and 14-kDa subunits. In the present work, oligonucleotide probes predicted from amino-terminal inhibin amino acid sequences have been used to isolate, from a porcine ovarian lambda gt11 cDNA library, clones encoding the 18-kDa subunit, or A chain, of inhibin. DNA sequence analysis showed that the inhibin A chain is initially synthesized as a larger precursor protein and is predicted to be a glycopeptide. Inhibin A-chain mRNA is present specifically in the gonads, and its synthesis can be induced by treatment of the animal with gonadotropins. The porcine probe was used to isolate a human inhibin A-subunit cDNA from a placental cDNA library. The human precursor is highly homologous to its porcine counterpart and is predicted to generate an 18-kDa glycosylated inhibin A subunit.