Treatment of T cells with staphylococcal enterotoxins in vitro is known to activate T cells in a subset restricted manner based on beta-chain variable region (V beta) gene expression. In particular, staphylococcal enterotoxin B (SEB) activates T cells bearing V beta 7 or V beta 8. We examined the ability of SEB to activate T cells in vivo. Treatment of C3H mice with doses of SEB ranging from 5 to 250 micrograms resulted in a dose-dependent activation of V beta 8+ T cells as reflected by increased interleukin 2 receptor (IL-2R) expression, proliferation to exogenous IL-2 and allogeneic cells, and production of gamma interferon. SEB also caused proliferation of the CD8+ subset of V beta 8+ cells in vivo. Thus, T-cell activation by SEB in vivo appears to be specific since V beta 2+ cells (non-SEB reactive) did not show increases in IL-2R expression similar to those seen with V beta 8+ cells nor did they proliferate. We then studied the ability of these activated cells to potentiate the immune response to a malignant progressor tumor. Treatment of C3H mice with 50 micrograms of SEB at the time of inoculation with tumor fragments resulted in a statistically significant decrease in the frequency of tumor outgrowth. These data demonstrate that treatment of C3H mice with SEB results in specific activation of V beta 8+ cells in vivo and that these activated cells are capable of preventing the outgrowth of a malignant tumor.