Superantigen-mediated T cell activation requires the participation of antigen-presenting cells (APC). Once superantigen has bound class II MHC molecules on the surface of APC, it then can interact with the T cell receptor to induce T cell activation. Superantigen-mediated T lymphocyte activation, along with its consequent cytokine production is thought to be the basis for the pathophysiology of conditions such as toxic shock syndrome, Kawasaki's disease and possibly rheumatoid arthritis. We examined the role of CD56+ NK lymphocytes in the interaction between superantigens and T lymphocytes. First, we found that a subpopulation of CD56+ cells freshly isolated from human peripheral blood expressed class II MHC molecules. The amount of HLA-DR expression varied between individuals, ranging from 9.3% to 37.7%. CD56+ (NK) cells were purified from the peripheral blood by cell sorting and were tested for their ability to support SEB-mediated T cell activation as assessed by surface expression of IL-2 receptor alpha-chain (CD25) on CD3+ lymphocytes. We observed that when enriched T cells were incubated with SEB in the presence of NK cells, there was a significant up-regulation of CD25 expression of the T cells. When HLA-DR+ cells were removed from sorted CD56+ populations, the remaining HLA-DR- NK cells were unable to support SEB-mediated T cell activation. Also, SEB up-regulated the expression of HLA-DR on CD56+ cells in peripheral blood mononuclear cell (PBMC) populations after 24 h of incubation, implying that the ability of NK cells to function as superantigen-presenting cells is up-regulated by superantigens themselves. Together, these data demonstrate for the first time that human CD56+ HLA-DR+ NK cells can function as superantigen-presenting cells, and imply that NK cells may be involved in the activation of non-specific T cell reactivity during early host defences against superantigen-elaborating microorganisms in vivo. Furthermore, the physical linkage of NK cells and T cells by the interaction of superantigen with HLA class II molecules and T cell receptors, respectively, may lead to NK cell activation and augmented lytic potential, helping to clear the body of superantigen-elaborating microorganisms.