A monoclonal antibody was produced to human peripheral blood T cells. This hybridoma antibody, termed OKT4, was reactive by indirect immunofluorescence with only 55-60% of the peripheral blood T cell population (OKT4+) and unreactive with normal B cells, null cells, and macrophages. The OKT4- T cell population contained the previously described TH2+ subset that has been shown to contain cytotoxic/suppressor cells. With cell-sorter separation of OKT4+ and OKT4- cells, it was shown that these T cell subsets were functionally discrete. Both gave proliferative responses with concanavalin A, alloantigens, and phytohemagglutinin although OKT4+ cells were much more responsive to the latter. OKT4+ cells alone responded to soluble antigens whereas OKT4- cells alone were cytotoxic after alloantigenic sensitization of unfractionated T cells. However, both OKT4+ and OKT4- cells were required during sensitization for optimal development of cytotoxicity. These data suggest that the OKT4+ subset represents a helper population and that the OKT4- subset contains the cytotoxic effector population. OKT4 could be a valuable reagent for determining alterations of these functional subsets in human diseases.