Twin blood group chimerism seems to be very rare in humans. The 30–40 previously reported cases usually were found by mere coincidence during routine blood grouping in hospitals or blood banks. Usually in these cases frank blood group mixtures of, for example, 50/50%, 25/75%, or 5/95% at most were seen. Smaller percentages are very difficult to notice during routine work‐up. Using a sensitive fluorescence technique (sensitivity >0.01%) we detected blood group chimerism in 32/415 (8%) twin pairs and 12/57 (21%) triplet pairs, respectively, which is a higher incidence than reported previously. © 1996 Wiley‐Liss, Inc.

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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).

A method for the investigation of erythrocyte chimaerism in patients after bone marrow transplantation (BMT) has been developed using fluorescent microspheres coated with anti-human IgG. Blood group antigens different in patient and donor were used as marker. The specificity and reliability of this method was evaluated measuring artificial mixtures of blood group positive and negative red cells. Red cell antigens tested were D, c, K, Fy^a, Fy^b, Jk^a, Jk^b, S and s. Mean linear regression lines for mixtures with percentages positive cells ranging from 0.01 to 1% and 1 to 100% were 0.91X-0.03 (r=0.99) and 1.00X-0.05 (r=0.99), respectively. The sensitivity level of the assay was one positive cell per 10,000 blood group negative cells. In negative control samples (n= 15) a mean percentage positive cells was found of 0.002 ± 0.004 (SD). Intra- and inter-assay coefficients of variation were 4.9 and 5.3% for mixtures of 10%, and were 11.1 and 24.6% for mixtures of 0.1% blood group positive cells. It is concluded that the described method can be used both to establish the take of donor marrow in an early phase after transplantation and to investigate mixed erythrocyte chimaerism long after BMT.

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