The lymphocyte proliferation is multicomponent mechanism of immune system reactivity. Many costimulatory factors take part in this process. Proinflammatory cytokines (TNF, IL-1 alpha and beta) enhance proliferation of activated lymphocytes. Female steroid hormones inhibit proliferation of mitogen and alloantigen-activated lymphocytes. The aim of this study was to investigate the effect of progesterone and 17 beta-estradiol on the costimulatory proliferative activity of proinflammatory cytokines in vitro. Female steroid hormones inhibit lymphocyte response to antiCD3 antibody. Progesterone had a stronger effect than 17 beta-estradiol (64 and 13% of inhibition respectively). 17 beta-estradiol enhanced the TNF costimulatory effect on the lymphocyte proliferation. Progesterone neutralized this TNF-induced effect and reverted it (inhibition of lymphocyte proliferation was enhanced in the presence of TNF). We found dominant inhibitory effect of progesterone on the TNF costimulatory activity when progesterone and estrogen were added simultaneously. Progesterone and 17 beta-estradiol downregulated costimulatory proliferative activity of IL-1 alpha or beta. Thus female steroid hormones had suppressive effect on the antiCD3-stimulated lymphocyte proliferation. They downregulated costimulatory proliferative activity of IL-1 alpha/beta and had opposite effect on TNF costimulatory activity. Our results suggest possible roles female steroid hormones as regulators on activity of proinflammatory cytokines and their functions in lymphocyte proliferation.