1. With purified blood group A and B specific substances from the gastric mucin of pigs precipitin reactions can be obtained in sera containing homologous isoagglutinins. By microquantitative precipitin technics the anti-A and anti-B content of normal and immune isoagglutinin-containing sera can be estimated on a weight basis.
2. There was a general correlation between antibody N precipitable from the sera of groups O and B by A substance and the agglutinin titer for A erythrocytes before and after immunization with blood group substances.
3. The reaction between A substance and its homologous isoagglutinin present in the sera of immunized individuals was of the same type as in other antigen-antibody systems.