Two brothers were transplanted with bone marrow which was depleted of mature T cells with monoclonal antibody CT-2 plus complement. One child died of sepsis due to candida present prior to transplant. The other is alive and well with full T and B cell reconstitution over 36 months after transplant. Thymus biopsy taken after transplant demonstrated normal morphology and cellularity. Portions of the marrow that were radiolabeled permitted an assessment of traffic patterns of aliquots that were injected intravenously and directly into the marrow space. The studies reported here document that a one haplotype mismatch is not a sufficient disparity to preclude both B and T cell reconstitution, and that monoclonal antibody plus complement is an effective method for T cell depletion.