SUMMARY The examination of a pigeon crop sac-stimulating hormone from flounder pituitaries by polyacrylamide gel electrophoresis is described. Elution of the proteins after electrophoretic separation permitted both the identification of the protein bands responsible for stimulating the crop and a comparison with the prolactin bands from rat and toad pituitaries, as well as from the pituitaries of several other species of fish. The nature of the crop-sac response is discussed and it is suggested that certain features, in particular the absence of fatty granules, indicate a difference between fish and tetrapod prolactin sufficient to warrant the adoption of an alternative name for the fish hormone.