Several techniques are available for the serological analysis of antigenic determinants on human hematopoietic progenitor cells (HPC). However, techniques for the recognition of cellularly defined antigens on such progenitor cells have not yet been described. We therefore developed an in vitro cellular cytotoxicity assay, with bone marrow cells as target cells. In this assay specific cytotoxic T lymphocyte (CTL) lines are used as effectors for cell-mediated cytolysis of bone marrow mononuclear cells that express the antigens for which the CTLs were primed in a mixed lymphocyte culture. As a model we used CTL lines against HLA-A2 or -B7 determinants. By using effector-target ratios varying from 1:2 to 4:1, 4 hr of incubation of these CTL lines with bone marrow mononuclear cells from HLA-A2 or -B7 positive donors resulted in a specific dose-dependent growth inhibition up to 100% of myeloid (CFU-GM), erythroid (BFU-E), and multipotential (CFU-GEMM) HPC. In contrast no inhibition of HPC was observed using mononuclear bone marrow cells from HLA-A2 or -B7 negative individuals as target cells. Experiments in which cell-cell contact was prevented showed that the antigen-specific lysis of HPC was dependent on intimate cell-cell contact between effector-CTLs and bone marrow target cells. Our results show that this cell-mediated cytotoxicity assay can be used as a sensitive and specific tool for the study of cellularly defined antigens on human hematopoietic progenitor cells.