Abstract.A canine red cell antigen, Tr, was defined by means of an immune canine antiserum produced by isoimmunization of a dog with canine red cells. The specificity of the antiserum was found to be identical to that of a naturally‐occurring canine antiserum which reacted specifically with human group A red cells after absorption with human group O and B red cells. Absorption and agglutination‐inhibition studies with human saliva showed that the canine red cell antigen Tr is related to the A antigen of human red cells; its representation on the canine red cell appears to be weaker than that of the A antigen of human A1red cells. The incidence of the Tr antigen on canine red cells is approximately 50%. No relationships were established between the canine A, B, C and D blood types and the ABH system of
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