An effective method for specific depletion of mature T lymphocytes from human bone marrow mononuclear cells (BMMC) with preservation of prethymic T cells and natural killer (NK) cells is presented. The BMMC were incubated with F101.01, a monoclonal antibody recognizing an epitope of the T-cell receptor-CD3 complex, and subsequently with immunomagnetic beads. Flow cytometric analysis demonstrated that mature T cells were efficiently depleted and that NK cells and prethymic T cells were preserved in the BMMC. Furthermore, T cell-mediated immune reaction as measured by thymidine incorporation after stimulation with phytohaemagglutinin was abolished, whereas NK activity as measured by 51Cr release using K562 as target cells was preserved. Recovery of colony-forming units of granulocyte-macrophages was 60-70%.