The aim of this study was to study the usefulness of erythrocyte antigen (EA) measurement to study engraftment after allogeneic HSCT. In all, 31 consecutive patients receiving HLA-identical bone marrow (BM) (n=13) or peripheral blood stem cells (n=18) were investigated. Apart from the ABO group, 15 EAs representing six minor blood groups were followed by the simple tube agglutination technique. A total of 20 (64.5%) patients received ABO-identical, eight (25.8%) received ABO minor and three (9.7%) received ABO major mismatched grafts. In all, 29 patients were followed for a median of 12 (6-16) months; 65% of the patients expressed donor type EA 1 month and almost all did so 6 months after transplant. Reticulocyte engraftment was significantly shorter than EA engraftment (median 18 vs 35 days) (P=0.001). Patients who received PB stem cells showed significantly faster EA and reticulocyte engraftment than patients who received BM stem cells (P=0.038 and 0.025). ABO compatibility did not have an impact on reticulocyte and EA engraftment (P=0.4 and 0.55). The earliest donor type EA detected was from the Rh and Kidd system. These data suggest that EA and reticulocyte assays are useful in monitoring engraftment.