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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.

The quantitative hemagglutination technic of Wilkie and Becker has been modified for the assay of saline and incomplete anti-D sera. By this method, the relative potencies of anti-D (Rho) sera were determined with an average error of 29 per cent (95 per cent confidence limits). The effect of duration of trypsin treatment upon the cell’s agglutination by incomplete antibody was investigated. The sensitivity of the technic in discerning small changes in agglutination was applied to the characterization of the prozone. The presence of a reversible equilibrium in the D-anti-D system has been established by two independent methods.