We have recently employed an extremely sensitive in vitro bioassay (based on the measurement of casein release from isolated mammary cells by reverse hemolytic plaque assay) to detect a lactogenic factor released from rat liver tissue. This material stimulates casein release in the absence of added prolactin (PRL) and is actually more potent than PRL in this regard. The substance possesses the following additional characteristics: (1) it is released from liver tisue of lactating but not virgin female or male rats and its production is clearly dependent upon PRL; (2) when tested together with PRL, its effects are additive rather than synergistic; (3) unlike PRL, it does not increase the proportion of mammary cells committed to casin release; (4) it is a peptide with a molecular weight of approximately 9400 daltons; (5) it is present in serum of lactating rats; (6) it can augment milk production by lactating rats in which PRL has been suppressed; and, (7) its act directly on pituitary cells to stimulate PRL release. These characteristics are consistent with the exciting possibility that a Liver Lactogenic Factor (LLF) may mediate the lactogenic component of PRL action. This functional relationship may be analagous to that which exists between growth hormone and the somatomedins. The primary objectives of the proposed research are: (1) to complete the biochemical characterization of LLF; (2) to elucidate the physiologic role of LLF; (3) to characterize the regulation of LLF secretion; (4) to evaluate the contribution of LLF to the suppression of gonadotropin secretion associated with hyperprolactinemia; and, (5) to determine whether LLF is present in mammalian species other than the rat. These objectives will be accomplished by utilizing standard methodologies for protein purification and structural analysis along with reverse hemolytic plaque assays and radioimmunoassays to analyze experiments conducted with both in vivo and in vitro model systems. The proposed studies will provide valuable information about the regulation of lactation. Moreover, this knowledge can eventually be used for identifying the causes of, and developing treatments for pathologies of prolactin and milk secretion in humans.