The presence of Ia-like antigens on human CFU-C and BFU-e is confirmed and a cell type that lacked immediate capacity for granulocytic colony formation but generated CFU-c after brief incubation in simple suspension culture is identified. This pre-CFU-c, and its immediate progeny, was extremely sensitive to killing by anti-Ia serum with complement. In contrast, anti-Ia serum plus complement treatment of human bone marrow, while eliminating 93%-97% of all CFU-c and BFU-e, did not prevent the rapid regeneration of these progenitor cells and their production for some weeks under the conditions of continuous marrow culture. These studies suggest that the human equivalent of the pluripotential stem cell can replicate for some weeks in culture and generate committed progenitors, such as CFU-c and BFU-e. Furthermore, it would appear that Ia-like antigen is absent on the pluripotential stem cell, is rapidly gained as commitment to the various progenitor cell types occur, and is subsequently lost as these latter undergo differentiation within the marrow.