Six patients with severe combined immunodeficiency were transplanted with bone marrow from their HLA haploidentical parents. T-lymphocytes were removed by complement mediated lysis with a monoclonal antibody (CAMPATH-I) followed by rosetting with sheep erythrocytes. The patients were pre-conditioned with marrow ablative chemotherapy, using busulphan and cyclophosphamide. There was graft take in five patients, with evidence of transient acute graft-versus-host disease only. Three patients are alive and well greater than 5 months after transplant, and one who is 12 months post graft has full reconstitution of cell mediated and humoral immunity, including specific antibody response to immunization. Transplantation across major histocompatibility barriers can be successfully achieved using this method of treatment of donor marrow.