The IBM 2991 Blood Cell Processor has been used to isolate a mononuclear cell (MNC) fraction from the marrow of 31 allogeneic donors. The MNC fraction was then incubated with a combination of two murine monoclonal antibodies MBG6 (CD6) and RFT8 (CD8) followed by two rounds of treatment with rabbit complement resulting in a marrow inoculum significantly reduced in the number of T-lymphocytes. We report here new specifications for the use of Ficoll-Metrizoate, the method used to calculate T-lymphocyte depletion and the details of our attempts to improve T-depletion. Following marrow transplantation with this T-depleted fraction, 29 patients are evaluable for engraftment, one patient failed to engraft and one died too early for evaluation. Twenty-two had no acute graft-versus-host disease (aGvHD), at a minimum of 60 d, six had grade I acute GvHD and one grade III. No correlation was found between the absolute number of MNC infused and time to engraftment, nor any relationship between the number of residual viable T-lymphocytes in the infused marrow and the incidence of GvHD, but the patient with the most severe aGvHD also had the highest number of T-lymphocytes infused.