Acute graft-versus-host disease (GVHD) following allogeneic bone marrow transplantation is the most significant limiting factor preventing the widespread application of transplant therapy in acute leukemia and aplastic anemia. GVHD is mediated by T cells that contaminate harvested marrow in proportions ranging from 5-50% of the mononuclear cell population. T cell depletion (TCD) of large volumes of human marrow by E-rosetting for 24 h at 4 degrees C with modified sheep erythrocytes achieves removal of greater than or equal to 97% of all T cells, as judged by cytofluorographic analysis of the T-depleted bone marrow population with a broad panel of anti-T cell monoclonal antibodies, and abrogates functional T cell activity. Although T-depleted bone marrow cell recoveries were 2 logs below total harvested buffy coat cell numbers, the TCD mononuclear population was more than 99% viable and was enriched twofold for Ia+ cells as judged by cytofluorographic analysis. This method is at least the equivalent of those employing lectin column or monoclonal antibody/complement lysis techniques and is simpler to perform. Successful engraftment of adult patients can safely be obtained with as few as 4 X 10(8) total mononuclear cells following the 24-h procedure suggesting that prolonged or repeated T-depletion procedures do not interfere with stem cell engraftment. Preliminary results suggest that this method of TCD may ameliorate GVHD in histoincompatible transplants.