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Summary A group AB mother (Mrs P.D.) gave birth to a group O female baby (C.D.). Extensive study of the blood group genetic markers in both the parents and the child, carried out on several occasions, showed nothing unusual outside the ABO system. Mrs P.D. then, gave birth to a second female baby who was also group O. Mrs P.D. had normal amounts of A, B, H and Lewis antigens in her saliva. The H, A and B agglutinability of her red cells was in the range of normal A2B group. This A2B blood group was characterized by very low A gene-specified glycosyltransferase activity in serum. Moreover this activity was undettable in red blood cell membranes. These results are discussed in the light of various hypotheses in order to explain this unusual transmission of ABO blood group.

SUMMARY The product of the A blood group gene in the erythrocyte membrane and serum from ‘weak A’ variants was investigated using low and high molecular weight acceptors and compared with common A1 and A2 blood samples. A reduced enzyme activity was detected, but only in some of the A variants, namely the A3 (eight out of eleven), Am or Ay samples. Other sera from A3 (three out of eleven), Ax, Aend or Ael individuals were apparently devoid of enzyme activity. A kinetic study by temperature inactivation of A blood group sera also showed that A enzymes are more labile than B enzymes from BI or BII sera. Moreover, in one case (A3 sample Del.), a very fast inactivation of the A enzyme was observed, suggesting the occurrence of a variant enzyme qualitatively different from the others so far studied. The erythrocyte membrane preparations from all A variants contained no detectable A enzyme activity except for the A1 and A2 samples, the former being three to five times more active than the latter. The B enzyme activities from four samples of B RBC tested were comparatively stronger than the A enzyme activity of A1 RBC. The results were discussed and it was suggested that the synthesis and/or the secretion of the A enzyme in the organism is not uniform from one tissue to another, but could depend on which of the ‘weak A’ alleles or modifier genes is involved.