We investigated whether staphylococcal exotoxins (SEs), in addition to their capacity to induce T-cell activation restricted by the T-cell receptor (TCR) beta-chain variable region, can deliver an activation signal to human T-cell clones through major histocompatibility complex (MHC) class II molecules. Eleven human T-cell clones (9 alpha beta TCR and 2 gamma delta TCR clones) of different antigenic specificities were tested for their capacity to proliferate in response to toxic shock syndrome toxin 1 (TSST-1) and two SEs, SEA and SEB. In the absence of accessory cells, only 4 alpha beta TCR clones were stimulated to proliferate, each by a single SE, and to mobilize intracellular free Ca2+ in response to that SE, events indicative of TCR engagement and, presumably, recognition restricted by the beta-chain variable region. In the presence of accessory cells, each of the 11 T-cell clones was stimulated to proliferate by any one of the three SEs tested. This apparently TCR-unrestricted SE-mediated polyclonal proliferation of T-cell clones occurred in the absence of an increase in intracellular free Ca2+ and was not dependent on the presence of MHC class II expression on accessory cells. In contrast, SE-mediated polyclonal proliferation did not occur in 3 alpha beta TCR clones derived from an MHC class II-deficient patient. Furthermore, all of the three SEs induced the proliferation of 4 natural-killer-cell clones, suggesting that expression of TCR/CD3 complex is not essential for SE-mediated polyclonal proliferation of activated lymphocytes. These results indicate that MHC class II molecules transduce activation signals to human T- and natural-killer-cell clones.