A three-year-old boy with severe aplastic anemia (HLA-A1,B8(Bw6), Cw7,DR3, MB2, MT2, SB4/A1,B8 (Bw6), Cw7,DR3,MB2,MT2,SB-) received a bone marrow transplant from a phenotypically HLA-identical, SB-compatible female unrelated donor. This donor was selected from eighteen HLA-A1,-B8,-blood donors after extended serotyping, mixed leukocyte culture testing and secondary proliferation assays with primed lymphocyte typing reagents specific for SB. Although patient cells proliferated well as responders in MLR, their stimulatory capability was greatly impaired. Because the patient had inherited the same serological HLA-D haplotype from each parent, it was concluded that a compatible unrelated donor must be homozygous for the same HLA-D antigens as the patient. This HLA-D homozygosity was demonstrated by the lack of MLR responses of both parents to stimulators from the donor. The SB typing results suggested SB compatibility because both the patient and the donor typed as SB4,-. Following bone marrow transplantation, there was rapid hematopoietic engraftment. The patient developed severe diarrhea caused by graft-versus-host disease of the gastrointestinal tract, which necessitated hyperalimentation. He is currently eighteen months posttransplant with full hematopoietic reconstitution and moderate chronic skin graft-versus-host disease.