Effects of progesterone on the immune system of different species are presented by a literature study. Elevated progesterone concentrations in blood result in more severe course and longer persistence of bacteria in infectious diseases and decrease of the clearance of antibody-coated erythrocytes in vivo. Progesterone suppresses specific components of the immune system and natural killer (NK) cell activity while it has a mainly positive influence on other nonspecific components. It suppresses blastogenesis and cytotoxicity of lymphocytes and increases synthesis of asymmetric antibodies without effector function to block fetal antigens. Progesterone increases the concentration of leucocytes in blood, random migration and chemotactic ability, synthesis of reactive oxygen intermediates (ROI) and antibody-dependent cellular cytotoxicity of polymorphonuclear leucocytes (PMN) and expression of complement receptors, synthesis of ROI and phagocytosis of peritoneal macrophages. In the uterus mainly immunosuppressive effects of progesterone are described (decreased migration of PMN into the gravid uterus, persistence of bacteria or transplants, synthesis of immunosuppressive uterine milk proteins, decreased random migration of PMN) except for an increased content of IgA in the secretory products indicating a significance of progesterone in the pathogenesis of endometritis mainly in the cow and bitch. No correlation could be found between blood concentrations of progesterone and placental retention in cattle. Progesterone takes part in avoiding immunologic aggression of the maternal organism against the semiallogenic conceptus by suppressing specific components of the immune system and NK cell activity. Positive effects on nonspecific components of the immune system could be regarded as compensatory mechanisms to maintain the efficiency of the complete immune system.