Human bone marrow was harvested from surgically resected bones of 25 patients and was tested for the presence of mature T cells. An average of 6.5% (±1.2% SE) of nucleated bone marrow cells formed spontaneous rosettes with sheep red blood cells. Functional T cells in bone marrow were also identified by characteristic responses to alloantigens and the T cell mitogens concanavalin A (Con A) and phytohemagglutinin (PHA). The ability of three monoclonal antibodies (OKT.3, Lyt-3, and (Leu-1) to lyse peripheral T cells in the presence of rabbit complement was examined. All three reagents were found to be specifically lytic for mature T cells in peripheral blood. One reagent (Leu-1) was selected for use in depletion of T cells in human bone marrow. Seven of 10 experiments performed showed sufficient T cell responses to be evaluable. In all of these experiments, a marked reduction of T cells and T cell functions was observed. On the average, E rosettes were reduced 89.2% (±3.0% SE) below medium controls while the mean PHA, Con A, and mixed lymphocyte culture (MLC) activity were completely eliminated to levels below background. In four experiments, colony-forming units (CFU-GM) in bone marrow were assayed following treatment with Leu-1 and showed a mean increase of 194% (±32% SE) over medium controls. Since mature T cells are thought to be responsible for graft-versus-host disease in allogeneic bone marrow transplantation, this method of T cell depletion may be useful for preparing marrow for human bone marrow transplants.