In order to confirm erythroid engraftment, we examined the changes in blood group antigens expressed on erythroblasts in the haematopoietic colonies of a bone marrow transplant recipient. A 44-year-old female with acute myelogenous leukaemia underwent an allogeneic bone marrow transplantation from her sister donor. Prior to the transplantation, the blood groups of both recipient and donor were analysed, and their M/N groups found to differ, the former being MN and the latter N. Bone marrow mononuclear cells were obtained from the patient on day 21 after bone marrow transplantation and cultured in semi-solid medium. Erythroblasts were collected from the erythroid bursts that had formed, and were subjected to flow cytometry using monoclonal anti-M and anti-N. Anti-N reactive cells accounted for 99.2% of those in the erythroid bursts, while only 0.3% of these cells were anti-M reactive. MN type erythrocytes in the recipient's peripheral blood had been replaced by N type and only 2.0% of erythrocytes were anti-M reactive on the 70th day after BMT. These erythroblasts and erythrocytes were phenotypically N, and originated from the donor haemopoietic stem cells with the success of bone marrow engraftment. From our findings, it appears that the M/N blood group antigens were produced by the erythroid cells themselves.