Four-tert-octylphenol (OP), an environmental pollutant, exerts apoptotic effects on cultured mouse splenocytes. Although OP binds to estrogen receptors, these apoptotic effects are not exerted by 17 beta-estradiol (E). It remained possible that OP might bind to estrogen receptors and subsequently exert apoptotic effects not exerted by E after it binds to the same receptors. It also remained possible that E-primed splenocytes might respond to OP differently than splenocytes not exposed to E. Thus, we investigated OP and E interactions on the viability of mouse splenocytes in culture. The total number of splenocytes (cells stained and not stained with trypan blue) was not altered or altered slightly after incubation with any agent for 24 h. Incubation of splenocytes in medium containing 5 x 10(-5) or 5 x 10(-7) M OP decreased the percentage of viable cells by only approx 47% and 25%, respectively. The addition of 0.8 x 10(-5) to 0.8 x 10(-9) M E to cultures was without effect or decreased the percentage of viable cells by only approx 5%. The addition of these concentrations of E simultaneously with or at 2 h after the addition of 5 x 10(-5) M or 5 x 10(-7) M OP to cultures did not interfere with the OP-induced decreases in cell viability. By contrast, incubation of splenocytes in medium containing E for 2 h prior to the subsequent addition of either dose of OP blocked the OP-induced decreases in cell viability in a dose-response manner. There was a marked reduction in the percentage of viable cells (70%) when splenocytes were incubated with 0.5 x 10(-5) M dexamethasone. The addition of 0.8 x 10(-5) M E at 2 h prior to the addition of dexamethasone did not prevent the decreased cell viability. Incubation of cells in medium with 0.8 x 10(-5) M testosterone caused a small decrease in splenocyte viability similar to that observed with E. However, unlike E, the addition of testosterone at 2 h prior to the addition of 5 x 10(-5) M OP did not prevent the OP-induced decrease in cell viability. These data suggest the presence of estrogen receptors in some splenocytes. They also suggest that if OP binds to these estrogen receptors or other receptors in the absence or initial presence of E, the resulting effect is toxic to the cells. By contrast, exposure of splenocytes to E prior to their exposure to OP can prevent the toxicity of OP.