A key question in biology is whether all cells of a given ‘cell type’ within an individual have more or less the same phenotype, especially in relation to nonimprinted autosomal loci. Some studies have shown differential allelic expression of autosomal genes to confer phenotypic variability at the individual cell level. Here, we report the amount of A and B histo-blood group antigens, products of classic examples of codominant alleles, in individual red blood cells (RBCs). Using immunofluorescence with Cy3-tagged and FITC-taggedantibodies, we quantified the levels of these antigens in 2512 RBCs from 24 individuals in the AB blood group. When these data were fit to a normal distribution, we could detect four groups: showing normal distribution for both antigens, either antigen, and neither antigen. Surprisingly, very few samples showed a significant positive correlation between the amounts of A and B antigens on individual RBC; in fact, the ratio of antigen A to antigen B in the entire set of samples spanned over five orders of magnitude. This variability in the amount of antigens A and/or B, combined with a lack of correlation between the amounts of these two antigens, resulted in unique staining patterns ofRBC, generating widespread mosaicism in the RBC population of AB blood group individuals.