Using immunoelectron microscopy, the distribution of the H antigen sites on human erythrocytes was observed in 40 samples of adult, newborn and fetal blood of different ABO phenotypes. The attached ferritin particles indicating the H antigen sites conspicuously varied in number from cell to cell in every specimen. The number of H antigen sites per single red cell was estimated on an average for each sample as follows: O, 3 X 10(5); B, 2 X 10(5); A1, 1.5 X 10(5); A1B, 10(5); A2B, 1.5 X 10(5); Ax, 2.5 X 10(5); AxB, 10(5); Bm, 4 X 10(5); Bw(leukemia), 4 X 10(5); O(newborn), 2.5 X 10(5); B(newborn), 3 X 10(5); A1(newborn), 1.5 X 10(5); A1B(newborn), 2 X 10(5); A1B(fetus), 10(5). The cells in each sample were divided into six cell-populatons according to the number of H antigen sites present. The ratios of distribution of such cell populations are compared for all samples.