The reactions of hog, human, and horse blood group A substances with antibodies produced on injection of these substances into individuals of blood groups B and O have been studied by hemagglutination inhibition and by quantitative precipitin technics.

Species differences in reactivity with antisera exist among these blood group A substances. Hog and human A substances are very closely related chemically as evidenced by their extensive cross-reaction while horse A substance crossreacts much less with antisera to both hog and human A substances.

Analysis for hexosamine of specific precipitates of hog and human A substances and antibody to human A substance formed as a consequence of heterospecific pregnancy has shown that essentially all of the glucosamine of the blood group substance is precipitated.

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