The pituitary hormone prolactin was discovered by Stricker and Grueter in 1928. These investigators observed that extracts of bovine pituitary glands induced lactation when injected into pseudopregnant rabbits. The effect of prolactin on the Columbid crop was first reported by Riddle and Braucher in 1931. This effect of prolactin served for many years as the method of choice for the bioassay of this hormone. It is thus clear that prolactin has a number of proliferative and differentiative actions on many cell types derived from a number of animal species. Most experimental work concerned with the molecular processes by which prolactin carries out its effects has been accomplished employing the mammary gland as the target organ. The interaction of prolactin with the dissociable binding sites appears to initiate at least certain effects of prolactin in target cells. Antibodies to the prolactin-dissociable "receptors" cause prolactin-like responses on several molecular processes in prolactin target cells.