We describe a patient with an exceptionally long-lasting red-cell aplasia of 330 days following ABO-incompatible bone marrow transplantation (BMT). Before BMT, the anti-B titre was high, 1:1,024, and it was only temporarily reduced by extensive plasma exchange. The anti-B titre remained above the level of 1:64 for 270 days, and host-derived isoagglutinin could still be detected 3 years after BMT. In vitro bone marrow cultures during the red-cell aplasia showed greatly reduced numbers or total absence of CFU-E, while the number of BFU-E colonies was only moderately subnormal. Six years after BMT, bone marrow and peripheral-blood cell counts are normal.