1. Flocculation reactions of anti-erythrocyte sera on emulsions of alcoholic extract of blood are described.

2. The reactions are markedly species-specific.

3. Besides the homologous reactions certain others—"heterogenetic"" ones—have been observed, and in this way the existence of new examples of heterogenetic antibodies has been demonstrated.

4. Group-specific substances can be extracted from human erythrocytes with alcohol and demonstrated by flocculation with group-specific immune sera.

5. A conception of the structure of cellular antigens based upon the known facts, is presented.

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