Normal human bone marrow contains cells capable of forming myeloid colonies (CFU-DG) in fibrin clots in diffusion chambers placed in the peritoneal cavity of cyclophosphamide-treated mice. Evidence has accumulated indicating that these colony-forming cells represent an earlier stem cell than the granulocyte-monocyte precursor cell (CFU-C) assayed in soft agar. We provide data showing that these stem cells express the "Ia-like" or DR antigen. In the presence of rabbit Ia antiserum at a titer of 1:300, all CFU-DG were inhibited. Cytotoxicity was complement-dependent. Data are also presented that suggest that the megakaryocyte stem cell also expresses the Ia antigen.