A splenocyte culture system supplemented with liver microsomes was developed to detect immunotoxic chemicals which require metabolic activation using cyclophosphamide as a positive standard. When liver microsomes were added to splenocyte cultures isolated from female B6C3Fl mice, the proliferation of splenocytes by lipopolysaccharide (LPS) was increased and the proliferation by concanavalin A (Con A) was decreased. However, when compared with each corresponding control, cyclophophamide was successfully activated to metabolites capable of suppressing Iymphoproliferative responses. This suppression was clearly dependent upon the amounts of microsomes added and/or the concentration of cyclophosphamide exposed. In these cultures, the proliferation of splenocytes was suppressed when the cells were exposed to cyclophosphamide on the day of culture initiation. On the other hand, microsome was responsible for the increase in LPS mitogenicity and NADPH was responsible for the decrease in Con A mitogenicity. Finally, our present culture system was compared with the hepatocyte-splenocyte coculture system which we had developed earlier. We found that the hepatocyte-splenocyte coculture was better able to activate cyclophosphamide to metabolites capable of suppressing the antibody response to sheep erythrocytes. Although our present culture system was relatively poor to activate cyclophosphamide in cultures for antibody response, it will be useful as a simple screening method to detect suppression of certain in vitro immunotoxic parameters like LPS mitogenicity by chemicals which require metabolism.