The chick oviduct contains two distinct forms of the progesterone receptor, termed progestophilins A (Mr = 79,000) and B (Mr = 117,000). Although these two hormone-binding proteins differ significantly in physico-chemical characteristics, a good deal of similarity exists between these two molecules. Thus, both proteins display identical hormone-binding kinetics and steroid specificity. The hormone-binding fragments obtained after the action of an endogenous oviduct Ca2+-activated protease are indistinguishable as analyzed by gel filtration chromatography. Both the A and B proteins are capable of binding to DNA-cellulose, although they elute at very different salt concentrations when subjected to gradient elution. In spite of these similarities all attempts to demonstrate the conversion of progestophilin B to A or to show a common precursor to both have been unsuccessful. Therefore, either the A and B receptor proteins are separate products of closely related genes or the conversion of one form to another occurs very rapidly either in vivo or in vitro.