The serum of J.R. contains anti‐Wrband her red blood cells are of the phenotype Wr(a‐b‐). Evidence was obtained that suggested that her cells totally lack normal MN and Ss sialoglycoproteins (SGPs), and carry instead an abnormal SGP, which is likely to be a hybrid SGP resembling the MN SGP in its outer portion and the Ss SGP in its inner portion. Although the apparent hybrid SGPs of J.R. and theMiVhomozygote are virtually indistinguishable in terms of their electrophoretic mobility (apparent molecular weight 40,000) and staining characteristics, they are not identical. That of J.R. is associated with a weak M and increased S antigen, while that of theMiVhomozygote is associated with a very weak N, a greatly exalted s, and the rare antigen, Hil. Like the study on the blood of theMiVhomozygote, serologic studies on the red blood cells of J.R. have revealed considerable heterogeneity of what have previously been called the Enaantigen and anti‐Enaant