Summary.A simple and sensitive flow cytometric method is presented for the quantitation of erythrocyte subpopulations. Cells were indirectly labelled using human antisera (mostly crude) and fluorescein isothiocyanate (FITC)-conjugated anti-human IgG F ab . The method was evaluated by analysing artificial mixtures of blood group antigens A, B, D, E, c, K and Fy a . Intra-assay coefficients of variation were found to be 7 . 2% for 1% D-positive mixtures and 2 . 2% for 10% D-positive mixtures; the inter-assay coefficients of variation were 9 . 5% and 6 . 0% respectively. The sensitivity of the method was found to be 0 . 31%. The method has shown to be suitable for the routine follow-up of patients after allogeneic bone marrow transplantation (BMT).