The studies presented herein have evaluated both the specificity and cellular basis of cell-mediated lympholysis (CML) in man. An efficient and quantitative 51Cr release assay was utilized to study the role of highly purified human T and B cells in CML. After in vitro sensitization human T cells develop the capacity to kill specifically allogeneic cells to which they were sensitized. In contrast, B cells were neither triggered to proliferate nor activated to kill allogeneic targets. B cells were not activated to kill even when sensitized in the presence of potentially "helper" T cells, nor did they block T cells from killing during the effector phase. Cell-free supernatants taken from active in vitro sensitization cultures were not lympholytic and did not modulate T cell killing. Hence, these studies show that both the afferent and efferent phases of human CML are T cell functions.