Clinical and experimental evidence supports the hypothesis that some steroidal drugs with androgenic effects might influence the immune system. The present study investigated whether gestrinone is able to affect macrophage and lymphocyte activity in vitro. Macrophage function was determined by phagocytosis of fluorescent microspheres, whilst lymphocyte proliferation was assessed by cell counting. Macrophage phagocytosis was evaluated after an overnight incubation in the presence or absence of gestrinone at serial dilutions; lymphocyte proliferation was detected in basal conditions and after stimulation with Concanavalin A (Con A) in the presence or absence of gestrinone. The results of this study showed that gestrinone significantly inhibited macrophage phagocytosis at the concentrations of 10(-8), 3 x 10(-8) and 10(-7) M. Furthermore, a significant suppression of lymphocyte blastogenesis was observed when lymphocytes were incubated with gestrinone at the concentration of 10(-7) M for 6 days. The biological significance of gestrinone as an inhibitor of immune functions under experimentally defined conditions is discussed in relation to its potential mechanism for fertility enhancement.