Chlorpromazine (CPZ) has potent immunomodulatory effects in vivo; it induces humoral autoimmunity in up to 50% of patients, inhibits delayed-type hypersensitivity reactions, and suppresses lethal immune hyperactivation in animal models of septic shock. Here, we show that in an in vivo model of acute superantigen-driven immune activation, CPZ independently down-regulates the production of various T cell-derived lymphokines (IL-2, IFN-gamma, IL-4, TNF, and GM-CSF) and up-regulates the secretion of IL-10. Whereas only low, if any, serum IL-10 levels are detectable by ELISA after injection of CPZ, bacterial LPS, or staphylococcal enterotoxin B (SEB) alone, simultaneous administration of CPZ + LPS or CPZ + SEB causes a significant increase in IL-10 production in vivo. CPZ-mediated amplification of the SEB-driven CPZ secretion is accompanied by an enhanced IL-10 mRNA accumulation, as shown by PCR analysis and in situ hybridization. Determination of IL-10 production in mice lacking T cells, B cells, or phagocytes revealed that SEB + CPZ-induced IL-10 was produced by phagocytic cells, but not by lymphocytes, a finding that is in accord with the distribution of splenic cells transcribing the IL-10 gene in response to SEB + CPZ. Moreover, these data indicate that bacterial superantigen can directly stimulate tissue phagocytes, even in the virtual absence of T lymphocytes. The blockade of dopamine type 1 (D1) but not type 2 (D2) receptors abolishes the CPZ effect on IL-10 production. Inhibition of Th1 and Th2 lymphokine production by CPZ is not mediated by dopamine receptors and is independent of IL-10 up-regulation. These findings may explain the mechanism by which CPZ and related drugs enhance humoral autoimmune reactions, block cellular immune responses, and prevent lethal septic shock in vivo.