Forty-six infants and children suffering from either inherited immunodeficiency disorders (Wiskott-Aldrich syndrome, functional T-cell immunodeficiency with or without HLA class II expression deficiency), malignant osteopetrosis, or Fanconi's anemia received HLA-nonidentical bone marrow transplantation (BMT) from related donors. Bone marrow was T-cell depleted to reduce the risk of graft-versus-host disease (GVHD). To prevent graft failure, a mouse monoclonal antibody specific for the CD11a-lymphocyte function-associated antigen 1 (LFA-1) molecule was infused into the patients. Eleven patients received five infusions of 0.1 mg/kg every other day from day -3 to +5. Thirty-five patients received 0.2 mg/kg daily from day -3 to +6. The overall sustained engraftment rate was 72% instead of 26.1% in a historical control group of 24 patients similarly treated except for the infusion of the anti-LFA-1 antibody. No late rejection occurred. The T-cell depletion method (E-rosetting or Campath IM plus complement) resulted in different rate of engraftment (83.3% v 57.9%, respectively, P = .05). Engraftment rate was slightly but not significantly influenced by the degree of HLA incompatibility between donor and recipient. Acute GVHD of grade II or more occurred in 35.5% of the patients and the rate of chronic GVHD was 12.9%. The overall actuarial survival rate with a functional graft is 47.3% with a mean follow-up of 28.0 months for patients with immunodeficiency and osteopetrosis, while none of the four patients with Fanconi's anemia survived. The development of full T-cell functions took on the average 6 months and of full B-cell functions 10 months. Significant infectious problems developed in the majority of the patients during the posttransplant course. Epstein-Barr virus-induced B-cell proliferative syndromes were observed in seven patients, six of whom had Wiskott-Aldrich syndrome. Correction of immunodeficiency was comparable in terms of kinetics and quality with that observed in patients with severe combined immunodeficiency undergoing HLA-nonidentical BMT. Correction of osteopetrosis appears not to be different from what has been observed after HLA-identical BMT. The in vivo use of an anti-CD11a-LFA-1 antibody as an additional immunosuppressive therapy in HLA-nonidentical BMT may thus promote engraftment and survival with correction of the primary disease in a significant number of patients with life-threatening immunodeficiency and osteopetrosis, but not with Fanconi's anemia.