Cytochrome P-450-mediated drug biotransformation is depressed by many immune stimulants. We have shown that such depression caused by the immune stimulant dextran sulfate is mediated by Kupffer cells. The purpose of this study is to determine if other cells of the immune system or a cellular product such as interferon are involved in the depressive action of dextran sulfate on drug metabolism. Plasma samples taken from mice treated with dextran sulfate contained no detectable interferon, yet hepatic cytochrome P-450 was significantly depressed, suggesting that interferon did not mediate the depression of drug metabolism by dextran sulfate. Administration of cyclophosphamide or antilymphocyte serum to mice prior to dextran sulfate to markedly decrease lymphocyte populations did not prevent the depressive actions of dextran sulfate on cytochrome P-450, suggesting that the lymphocyte population was not involved in the dextran sulfate mediated depression. Preincubation of dextran sulfate with peritoneal macrophages prior to incubation with hepatocytes significantly depressed hepatocyte cytochrome P-450 content, while dextran sulfate alone had no direct effect on hepatocyte cytochrome P-450 content. These results further support the hypothesis that macrophages play a major role in the depression of cytochrome P-450 by dextran sulfate.