Rh-positive red cells (RBCs) that fail to agglutinate with commercial anti-D reagents by the tube method are considered to have the Du phenotype. The quantities of D epitopes on such RBCs have been measured previously in one kindred. The authors report on the number of D epitopes on Du RBCs of 23 unrelated individuals, as calculated by Scatchard's analysis. Cell-bound anti-D was measured by an automated antiglobulin consumption technique. On the average, RBCs of the Rh phenotype CcDue had a mean of 1568 +/-1220 (n = 12) D epitopes per cell. The relatively large range of values in this group implies a heterogeneous genetic background. The lowest number of D epitopes, 285 per cell, was observed on the RBCs of one individual who was apparently homozygous for C. In this case, the D antigen was detected only by adsorption/elution tests. RBCs of the phenotype cDuEe had a mean of 775 +/- 378 epitopes per cell (n = 8), and those from two individuals with phenotype cDue had 2840 and 1560 D epitopes, respectively. Thus, on the average, RBCs with the Du phenotype bear about 10 to 20 times less D antigen than normal Rh-positive RBCs. It is suggested that the low D antigen density of Du red cells may account for their poor immunogenicity.