The pyrogenic response to supernatant fluids obtained from human peripheral blood mononuclear cells (PBMC) stimulated with staphylococcal enterotoxin A (SEA) was characteristic of a response to an endogenous pyrogen in that it was brief and monophasic and was destroyed by heating supernatant fluids at 70°C for 30 min. The febrile responses were in parallel with the levels of interleukin-1 (IL-1), tumor necrosis factor (TNF), interferon- (IFN-), IL-2, and IL-6 in supernatant fluids obtained from PBMC treated with SEA. Both the pyrogenicity and the levels of IL-1, TNF, IFN-, IL-2, and IL-6 in supernatant fluids started to rise at 6 to 18 h and reached their peak levels at 24 to 96 h after SEA incubation. Both the fever and the increased levels of IL-1, TNF, IFN-, IL-2, and IL-6 in supernatant fluids obtained from the SEA-stimulated PBMC were decreased by incubating SEA-PBMC with anisomycin (a protein synthesis inhibitor), aminoguanidine (an inhibitor of inducible nitric oxide synthase [NOS]), or dexamethasone (an inhibitor of NOS). The febrile response to supernatant fluids obtained from the SEA-stimulated PBMC was attenuated by adding either anti-IL-1, anti-TNF-, or anti-IFN- monoclonal antibody (MAb) to supernatant fluids. The antipyretic effects exerted by anti-IL-1 MAb were greater than those exerted by anti-TNF- or anti-IFN- MAb. The data suggest that SEA acts through the NOS mechanisms in PBMC to stimulate synthesis of pyrogenic cytokines (in particular, the IL-1).