The staphylococcal toxins are responsible for a number of diseases in man and other animals. Many of them have also long been known to be powerful T cell stimulants. They do not, however, stimulate all T cells. On the contrary, each toxin reacts with human T cells bearing particular Vβ sequences as part of their receptors for major histocompatibility complex protein-associated antigen. The specificity of these toxins for Vβs puts them in the recently described class of superantigens and may account for the differential sensitivity of different individuals to the toxic effects of these proteins.