An 11-yr, 9-mo-old girl with a 5-hr history of severe aplastic anemia unresponsive to androgen or steroid therapy was treated with a bone marrow transplant from her 7-yr-old, HLA-MLC identical, ABO incompatible, male sibling. Donor neutrophils were positive for the NA1 antigen. The patient's pretransplant serum contained a neutrophil specific antibody, anti-NA1, reactive against donor neutrophils, which was demonstrable by both granulocyte agglutination and granulocyte cytotoxicity assays. The transplant preparative regimen consisted of cyclophosphamide, 50 mg/kg/day for 4 days, and total lymphoid irradiation (750 rads, 26 rads/min by 4 meV linear accelerator). A two plasma volume exchange (4,800 ml) by continuous flow centrifugation with an Aminco cell separator was performed one day prior to transplant because of ABO incompatibility. Following plasma exchange, anti-NA1 cytotoxic titer, 1:8 preexchange, was no longer detectable; anti-NA1 agglutinating titer had only decreased from 1:64 to 1:32. She experienced no adverse reaction to transplantation of 4.8 x 108 nucleated cells/kg. Marrow engraftment was demonstrable by Day 14 by steadily increasing leukocyte and platelet counts, red cells of donor ABO group, and bone marrow chromosomes showing a normal male, 46 XY karotype. This case of successful bone marrow engraftment without delayed neutrophil recovery in the presence of a neutrophil specific antibody NA1 suggests that neutrophil specific antigens are not functionally present on the pluripotent stem cell. Histoincompatibility for neutrophil specific determinants need not eliminate the possibility of bone marrow transplantation for aplastic anemia between siblings identical for the major histocompatibility loci.