Intraluminal Prolactin (PRL) binding activity in dorsal and lateral prostate may also have relevance to the observations from other laboratories of immunoreactive PRL accumulation in seminal fluid, which is derived apparently from sex accessory structures rather than spermatozoa or testicular fluid, and that intraluminal PRL may play a major role in spermatozoan capaci-tation. The fact that prostate tissue specimens retain PRL binding activity after having undergone tissue processing seems remarkable. Immunohistochemistry has the distinct advantage of providing an in situ cellular and subcellular localization of PRL target cells within a complex tissue such as prostate and, in so doing, can provide important physiologic insights. The effects of ethanol on membrane fluidity were entirely consistent with those observed for PRL binding activity. The fact that cleaved PRL is biologically active strengthens the suggestion that limited proteolysis of PRL in target tissue has biological relevance.