The log-probit hemagglutinins assay has been modified and applied to a study of human group A and AB red cells reacted with homologous stimulated group B sera. Qualitative differences among the A antigens were shown by the characteristic slopes produced by each subgroup. The decreasing progression in slope of the A cells was of the order: A1 > A1B > A2 > AintB > A2B > A3. The findings of differences in the mode of reaction of A and AB cells along with that previously reported for the B and A1B cells lends support to the theory of Morgan and Watkins that the substance produced by the gene in the heterozygote differs from that of either gene in the homozygous condition. Significant quantitative differences in the content of agglutinogen were found among the cells of A1 individuals with stimulated group B sera. Three percentage grades of A1 have been recognized which are not related to genotype.