IN view of the stress laid by many anthropologists on the significance of the blood groups in the racial classification of man, attention may be directed to the results of certain investigations of the weak Areaction found in some cases of the group A Bby G. L. Taylor, R. R. Race, Aileen M. Prior and Elizabeth W. Ikin (Brit. Med. J.,Feb. 24, 1940). It had already been shown that in some ABcases the factor is partially dominant to and obscures the Aantigen; and that while there tends to be some weakening of the reaction in group AB,it is nothing like so marked or so important as is the suppression of A.As there are two types of Aantigen, Ax and A-a stronger and a weaker, the suppression by of an already weak A2 in the group AJBmay result in it only being possible to detect the Afactor with powerful anti-4 serum, the cells otherwise being diagnosed as group B.Following on the observations recorded, it is shown that the order of decreasing strength of reaction is A, AXB, A2, AJB.Further, as the serum from quite a number of A2Bcases contains the antibody OL19which reacts with Axbut not with A2-cells, unless appropriate measures are taken, a reaction will result which confirms the diagnosis of group B.In view of the significance attached in anthropological classification to the distribution both within a given population and geographically of the elements Aand B,it is of interest and importance to note that cases diagnosed as ?have been found on re-examination to belong to group AB,and an inspection of published figures from all parts of the world shows that in a large proportion of the series there is a real deficiency of the numbers in group AB.It is possible that herein lies a clue to the explanation of certain anomalies and apparent irregularities to be noted in studies of the groups as racial elements.