Although bacterial superantigens have been well characterized as potent stimulators of T cells, their role in natural killer (NK)-type cells remains largely unknown. In the present study, we examined the effect of bacterial superantigens on mouse liver NK cells and NK1.1 Ag ุ (NK1 ุ ) T cells. C57BL/6 mice were intravenously injected with staphylococcal enterotoxin B (SEB) or streptococcal pyrogenic exotoxin A (SPE-A), and mononuclear cells (MNC) of various organs were obtained from mice 4 hours after being injected with superantigen. MNC were cultured for 48 hours, and interferon gamma (IFN-โฅ) levels of supernatants were measured. The antitumor cytotoxicities of the liver and spleen MNC were also evaluated 24 hours after the mice were injected with superantigen. Liver MNC produced more IFN-โฅ than did splenocytes, and peripheral blood and lung MNC did not produce any detectable IFN-โฅ. In addition, liver MNC acquired a potent antitumor cytotoxicity by the SEB injection, and both NK cells and NK1 ุ T cells but not cluster of differentiation (CD)8 ุ T cells were responsible for the cytotoxicity as demonstrated by either in vivo or in vitro cell depletion experiments, and the NK-type cells were partly responsible for the increased serum IFN-โฅ. Activation of liver NK-type cells was also supported by the fact that liver NK cells proportionally increased and NK1 ุ T cells augmented their CD11a expressions after SEB injection. The pretreatment of mice with anti-IFN-โฅ Ab and/or with anti-interleukin-12 (IL-12) Ab diminished the SEB-induced cytotoxicity of liver MNC. Furthermore, the in vivo depletion of Kupffer cells decreased the SEB-induced cytotoxicity of liver MNC. Consistent with these results, liver MNC stimulated with superantigens in the presence of Kupffer cells in vitro produced a greater amount of IFN-โฅ than did the liver MNC without Kupffer cells or splenocytes. Our results suggest that bacterial superantigen-primed Kupffer cells produce IL-12 and other monokines, while also nonspecifically activating both NK cells and NK1 ุ T cells to produce IFN-โฅ. (HEPATOL-OGY 1999;30:430-436.