SummaryProlactin (PRL), an endocrine hormone from the anterior pituitary, is also synthesized and secreted by activated immunocytes, mostly T cells and thymocytes. In vitro PRL acts as an autocrine or paracrine growth factor which regulates the proliferation of previously stimulated immunocompetent cells. PRL acts through its specific cell surface receptors expressed on different immunocytes including T cells, B cells, monocytes, NK cells and neutrophils. In vivo the immunoregulatory role of PRL is still the subject of intense investigation, especially in the conditions of stress and reproduction. The conflicting results suggest that PRL concentration, sex hormones and some other factors influence the immunomodulatory role of PRL.