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.