Long-term bone marrow cultures were established from single-cell suspensions of human bone marrow that had been treated with monoclonal antibodies and complement. Each treated cell suspension was evaluated for production of hematopoietic stem cells over 20 weeks. Treatment with antibody to HLA-DR (Ia), B1, J2, or J5 did not remove adherent cells including those differentiating to adipocytes in 17-hydroxycorticosteroid. In contrast, treatment with monoclonal antibody directed against human beta 2-microglobulin reduced adipocyte numbers by 100-fold and reduced the total adherent cell density over 70%. Cumulative total nonadherent cell and granulocyte-macrophage colony-forming units (GM-CFUc) production over 20 weeks was not significantly altered by one cycle of anti-Ia plus complement or up to three cycles of treatment with complement and anti-J2, -J5, or -B1. However, one cycle of treatment with anti-beta 2-micro-globulin depressed production of both GM-CFUc and nonadherent cells by over 100-fold compared to other treatment groups. While one cycle of treatment of anti-Ia and complement killed all detectable cells forming CFU-erythroid-granulocyte-megakaryocyte-macrophage, blast-forming units (erythroid), and GM-CFUc, GM cluster-forming cells survived. Treatment of marrow with three cycles of anti-Ia and complement removed all detectable GM colony- and GM cluster-forming cells; however, this marrow produced fewer cumulative Ia-positive GM-CFUc. Long-term bone marrow cultures may prove to be an interesting system for in vitro analysis of the effects of new immunotherapeutic agents including other monoclonal antibodies prior to clinical use.