T cell proliferation in response to stimulation with Staphylococcus enterotoxin A (SEA) requires accessory cells that express class II major histocompatibility complex (MHC) molecules. Murine fibroblasts transfected with genes encoding the α and β subunits of HLA-DR, DQ, or DP were used to show that the proliferative response of purified human T cells to SEA is dependent on class II molecules but is not restricted by the haplotype of the responder. Binding of fluoresceinated SEA to class II transfectants and precipitation of class II heterodimers with SEA-Sepharose show that the proliferative response is a result of SEA binding to class II molecules. The binding is specific for class II molecules and is independent of class II allotype or isotype. The ability of SEA to bind class II molecules may be a general characteristic of this class of antigens, now called "superantigens.".