A modified technique for photometric quantitation of haemagglutination was employed in experiments with anti‐A test sera and Arcell suspensions. The agglutinates and the free cells were separated by simple sedimentation. As an absolute separation was impossible in this way, no exact estimation of percentage agglutination could be made. But an estimation of the antibody dilution which removes 50 per cent of the erythrocytes by agglutination and accelerated sedimentation was a far more accurate value for the strength of the antibody than the serological titration commonly used.