Studies in our laboratory and others have recently shown that staphylococcal enterotoxin-derived superantigens stimulate proinflammatory cytokine gene expression in vitro. We have therefore investigated the ability of superantigens to induce leukocyte accumulation at extravascular sites in vivo using the subcutaneous air pouch model. Injection of staphylococcal enterotoxin A (SEA) induced a significant accumulation of leukocytes over basal levels in a time- and dose-dependent manner. It was also shown that superantigens are capable of inducing this response in mice depleted of CD4 T cells, as well as in severe combined immune-deficient and nude mice. These observations suggest that superantigens are capable of inducing leukocyte accumulation independently of the presence of T lymphocytes. Experiments were also conducted using mutant SEAs that have a reduced binding affinity for major histocompatibility complex (MHC) Class II molecules, as well as using MHC Class II-deficient mice. The results of these experiments indicated that MHC Class II molecules are not required for the observed effect of superantigens in vivo. Taken together, these results indicate, first, that bacterial superantigens promote inflammation in subcutaneous tissue in vivo and, second, the potential existence of a novel receptor for superantigens that mediates this subcutaneous inflammatory response.