In a previous study using a canine model, we reported that a certain anti-class II monoclonal antibody (MoAb H81.9), which recognizes an epitope formed by the alpha and beta subunits of HLA-DR, prevented long-term engraftment of autologous marrow cells if administered intravenously during the first 4 days after 9.2 Gy of total body irradiation. Another MoAb (B1F6), reactive with only the beta subunit of HLA-DR and -DP, had no adverse effect on engraftment, although both MoAbs detect antigens on hematopoietic long-term repopulating cells as determined from complement-mediated lysis experiments. In the present study, continuous exposure of unfractionated human marrow to MoAb H81.9 specifically inhibited the growth of primitive progenitor cells that require multiple hematopoietic growth factors for proliferation (high proliferative potential colony forming cells [HPP-CFC] and burst-forming units-erythroid [BFU-e]), but had no effect on more mature, single factor responsive (CFU-GM), progenitor cells. In contrast, MoAb B1F6 did not impair primitive progenitor cell growth cultured as unfractionated marrow. However, when cell dose-response experiments were performed using CD34-positive cells plated at low cell densities, the marked inhibitory effects of MoAb H81.9 on HPP-CFC and BFU-e colony formation were not seen. These findings suggest that MoAb H81.9 may not inhibit primitive hematopoietic cells directly, but rather indirectly through the action of potent mediators derived from other HLA-DR-positive marrow cells.