The reactivity of Rh positive red cells with saline anti-D sera has been investigated by means of quantitative hemagglutination methods. The inhibitory effect of C on the D antigen has been confirmed and the possibility of inhibition by this antigen in the cis position suggested. It is also suggested that the e antigen has suppressive effect on D. The presence of companion antigens results in a -D-> R2 > Ro > R1 progression of decreasing agglutinability. Within families differences in the agglutination behavior between homozygous and heterozygous D positive cells were found. The heterogeneity of this antigen was confirmed.