The cytokine production induced by a newly discovered streptococcal exotoxin, MF, and the pyrogenic exotoxins SpeA and SpeB was determined by in vitro stimulation of peripheral blood mononuclear cells (PBMCs) obtained from healthy blood donors. The induction and kinetics of interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-1 receptor antagonist, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, IL-10, gamma interferon, tumor necrosis factor alpha (TNF-alpha), TNF-beta, and granulocyte-macrophage colony-stimulating factor were studied at the single-cell level by use of cytokine-specific monoclonal antibodies and intracellular immunofluorescent juxtanuclear staining. The cytokine-producing cells, with the exception of IL-1-expressing cells, had a characteristic morphology generated by the accumulation of cytokines in the Golgi organelle. MF, SpeA, and SpeB induced a massive gamma interferon and TNF-beta response in 10 to 16% of the PBMCs after 48 to 96 h of cell stimulation. In contrast, IL-2 and TNF-alpha production was detected in only 1 to 3% of the PBMCs. The induction of a lymphocyte TH2 phenotype response, including production of IL-3, IL-4, IL-5, and IL-10, was weak. However, the monokines, IL-1 alpha, IL-1 beta, IL-1 receptor antagonist, and IL-8, were consistently found and gradually produced, peaking at 24 h in approximately 5 to 8% of the PBMCs. MF showed extensive cytokine- and proliferation-inducing capacities equal to those of SpeA and SpeB, which suggests that MF is also a superantigen. A marked interindividual variation could be noted both in the proliferative response and in the cytokine induction of lymphocytes isolated from different individuals, which may be one explanation for the varying clinical severity noticed during group A streptococcal infections.