The amount of blood group A substance on platelets is proportional to the amount in the plasma.

To investigate whether platelet ABO antigens are intrinsic to platelets or adsorbed from plasma. the amount of A-antigen on washed platelets was determined and related to the Lewis and secretor phenotypes, as these genes control the amount A, B. or H antigen present in plasma. The A-antigen on platelets was quantitated using a platelet-erythrocyte mixed agglutination assay in 28 A1 individuals of different Lewis and secretor phenotype. There was a direct correlation (r 0.6. p < 0.01 ) between the agglutination score and the predicted amount of A-antigen in the plasma. The highest agglutination scores were obtained with platelets from Le(a b ) secretors, and the lowest score with platelets from Le(a b ) nonsecretors. The quantity of A-substance on platelets from A1 individuals was significantly higher than from A2 individuals of the same Lewis and secretor phenotype (n = 1 7; p < 0.01). Group 0 platelets were incubated with A-plasma in order to