An account is given of an experiment in which the use of a "blocking" procedure has enabled the site of action of the cross reacting antibody in group O serum to be shown to be homologous with the A and B antigens of the red cell. In another experiment cross reacting antibody was shown to behave asymmetrically. In both these experiments use was made of the mixed agglutination technic for directly demonstrating the adherence of two cells of differing specificity by cross reacting antibody.

The significance of the results of these experiments is discussed in the light of observations by other workers.

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