OBJECTIVE In vivo and in vitro studies have indicated that estradiol can affect cytokine production in different cell types. This study examines whether estradiol affects inflammatory cytokine production by murine splenic macrophages. METHODS Mouse splenic macrophages were first treated with 17 beta-estradiol, followed by lipopolysaccharide (LPS) stimulation. The production of cytokines by macrophages with or without estradiol treatment was determined at both the protein and mRNA levels. The nuclear factor-kB (NFkB) activity of activated mouse splenic macrophages was also evaluated by electrophoretic mobility shift assay. RESULTS Our results show that 17 beta-estradiol decreases LPS-induced IL-1 alpha, IL-6, and TNF-alpha production but not IL-10, IL-12, and macrophage inflammatory protein (MIP) production by splenic macrophages. Furthermore, inhibition of cytokine production by 17 beta-estradiol was associated with a decreased LPS-induced NFkB-binding activity. CONCLUSIONS Because cytokines are important mediators of immune function, the alteration of cytokine production by 17 beta-estradiol may thus have a profound effect on the outcome of immune response during inflammation.