Blood samples from 31 of 50 consecutive patients receiving bone marrow from an HLA-identical and mixed lymphocyte culture—nonreactive sibling were investigated for the presence of donor and autologous erythrocytes. Simple serological techniques using antigenic differences between donor and recipient and a blood transfusion policy taking these differences into account made this study possible. A total of 71% of the patients had donor erythrocytes demonstrable 4 weeks after bone marrow transplantation; almost all patients did so after 2 months. Disappearance or absence of donor red cells indicated poor patient prognosis. Persistence or reappearance of autologous erythrocytes in small percentages (0.05–10%) occurred without relapse of leukemia. Reappearance in high percentage (50–100%) indicated relapse.