T cell depletion using the murine monoclonal antibody (moAb) T10B9 is unique in that the T cell receptor (TCR)gamma delta bearing subset is relatively spared compared to the TCR alpha beta + subset. We evaluated the probabilities of engraftment, acute and chronic graft-versus-host disease (GVHD), relapse, and survival in 273 recipients of marrow T cell depleted using T10B9. Sixty-two patients received marrow from an HLA-identical sibling, 54 patients received partially matched related donor marrow and 157 patients received unrelated donor marrow. Limiting dilution analysis (LDA) was used to estimate total clonable T cell dose in all patients and a modified LDA using moAb-coated immunomagnetic beads was used to estimate TCR alpha beta +, CD4+, and CD8+ T cells in a subset of patients. TCR gamma delta + cell dose was estimated by flow cytometry. Cox proportional hazards regression models were used to assess the impact of T cell subset dose/kg of body weight on outcome. We found a significant association of TCR gamma delta + T cell dose (P = 0.004), but not TCR alpha beta + T cell dose or total clonable T cell dose, with the probability of engraftment. TCR alpha beta +, CD4+, CD8+ and total clonable T cell dose were significantly associated (P < 0.001) with the risks of grade 2-4 acute GVHD in recipients of marrow from related donors but not in recipients of marrow from unrelated donors. Neither total clonable T cell dose nor any T cell subset dose was found to be significantly associated with chronic GVHD, relapse or survival. The results confirm preclinical studies showing TCR gamma delta + T cells promote engraftment. TCR gamma delta + T cells are not associated with an increased risk of acute GVHD while TCR alpha beta T cells are associated with acute GVHD but not engraftment in recipients of marrow grafts T cell depleted using T10B9. These findings support the hypothesis that T cell subsets differentially contribute to alloengraftment and GVHD.