The overall objectives of this project include: 1) an evaluation of in vitro and in vivo assay systems to examine the effects of possible mammary gland factors on prolactin (PRL) secretion by rat and human mammotrophs; 2) study of interaction between these factors and other regulators of PRL secretion, and 3) the preliminary biochemical characterization of mammary factors affecting mammotroph activity. We have found that a potent, dose-related PIF (prolactin inhibiting factor) is present in rat serum. Preliminary evidence shows that the concentration of this factor may be dependent upon the physiological condition of the donor animal (lower PIF in rats bearing mammary tumors). Other data are consistent with the interpretation that this factor is a protein.