The reconstitution of hematopoietic cells and in vitro assays of immunologic function have been followed in leukemic patients after conventional bone marrow transplantation (BMT) (N = 34) and T-cell depleted BMT (N = 52) from human leukocyte antigen (HLA)-identical sibling donors. No effects of the T-cell depletion could be seen on the recovery of myeloid or lymphoid cells as measured by the day to engraftment or by the absolute number of cells through day 100. Normal numbers of lytically active natural killer cells returned the earliest and were rapidly followed in both groups of patients by the appearance of circulating B cells and normalization of the responses to B-cell mitogens. However, the recovery of normal T-cell proliferative responses were more delayed in recipients of T-cell depleted grafts. Significant quantitative differences were seen only during the first 3 months after transplantation. Neither the number of CD3+ T cells nor the ratio of CD4:CD8 positive cells differed markedly between the two transplant groups. Mitogen-induced immunoglobulin production by peripheral blood lymphocytes (PBL) from patients following T-cell depleted BMT was quantitatively less than that of conventional marrow recipients through the first year, with low normal IgM production reached by 4 to 6 months in both groups. IgG production reached low normal 7 to 9 months after conventional BMT but did not remain at this level until 1 year following either type of transplant. Assessment of the incidence of infections from the day the absolute neutrophil count reached 500 until day 180 after transplant revealed no significant differences between the two groups; indeed, the overall nonleukemic mortality was higher in the recipients of conventional bone marrow. Thus, in our series, the removal of mature cells from the marrow graft did not affect the rate or degree of recovery of myeloid and lymphoid cells but did affect the regeneration of in vitro T-cell dependent functions. We noted early quantitative differences and a delay in the normalization of the T-cell functions measured rather than prolonged absolute deficiencies. The in vitro deficiencies did not result in significant clinically apparent differences between the two groups.