The CD4 subset of cells displays helper/inducer activity and recognizes class II antigens of the major histocompatibility complex (MHC), while the CD8 subset recognizes class I MHC antigens and exhibits cytotoxic or suppressor function. Considerable functional as well as corresponding phenotypic heterogeneity exists within the two major T cell subsets. Although the CD8+ population contains pre-cytotoxic, cytotoxic, pre-suppressor and suppressor effector T cells, these distinctions still rest largely on the use of functional assays. Attempts have been made to define the CD8+ precursor of the killer cell with new monoclonal antibodies. But more precise phenotypic distinctions between the functional subpopulations within CD8+ cells will be needed. We have now developed a monoclonal antibody, anti-S6F1 which can distinguish killer effector and suppressor effector cells in CD8 lymphocyte populations. The cell-surface structure defined by this antibody comprises two glycoproteins with relative molecular mass (Mr) 180K and 95K respectively. Also sequential immunoprecipitation studies and two dimensional gel electrophoresis indicate that anti-S6F1 recognizes a novel epitope on the LFA-1 antigen.