The response of mouse T cells to the superantigen staphylococcal enterotoxin A (SEA) requires 1000-fold higher concentrations compared to human T cells. In order to develop a sensitive model for SEA studies in mice, the immunopharmacology has been studied in T-cell receptor (TcR) V beta 3 transgenic (TGV beta 3) and non-transgenic (non-TG) C57Bl/6 mice. The frequency of SEA-responsive T cells in the TGV beta 3 mice exceeded 90%, whereas a 10-fold lower frequency was seen in normal C57Bl/6 mice. Nanograms of SEA injected intravenously into TGV beta 3 mice induced strong cytolytic T lymphocyte (CTL) activity against SEA-coated major histocompatibility complex (MHC) class II+ B-lymphoma cells, whereas administration of 1000-fold higher amounts of SEA to non-TG littermates or normal C57Bl/6 mice induced only a moderate response. Kinetic analysis demonstrated that the CTL activity was more rapidly detectable in TG mice, but substantial levels were seen 2 days after SEA injection in both TGV beta 3 and non-TG mice. The cytotoxic T-cell response induced by SEA in TGV beta 3 and non-TG mice was completely MHC class II dependent, as SEA-coated MHC class II-transfected syngeneic B16 melanoma cells but not untransfected B16 cells were sensitive to lysis. Large amounts of tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) accumulated in serum of TGV beta 3 mice after injection of 10 ng SEA, whereas only marginal amounts were recorded in non-TG even after injection of 100 micrograms SEA. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis demonstrated that SEA-induced TNF-alpha and IFN-gamma mRNA reached maximal levels 1 hr after SEA administration in TGV beta 3 mice, whereas peak serum levels of TNF-alpha and IFN-gamma proteins were recorded after 2 hr. Comparison of the mRNA levels of a panel of cytokines in the TGV beta 3 and non-TG mice revealed that almost similar amounts of interleukin-1 (IL-1) were induced in both strains, whereas IL-4 was only detected at significant levels in the TGV beta 3 mouse. The results suggest that TGV beta 3 mice are suitable for studying in vivo immune responses to superantigens at concentrations comparable to the potent effects elicited in humans. Moreover, this model is useful for detailed studies on the dynamic regulation of T-cell activation and anergy induced by superantigens.