Abstract. Agglutination‐inhibition studies have shown the presence of blood group substance H activity in canine saliva, and of blood group substance A activity in the saliva of Tr+ dogs. H substance could not be demonstrated on canine red cells, indicating that the cellular A‐like antigen, Tr, is formed independently of H. Nonsecretor status, and an antigen comparable to the human B antigen, have not been detected.

SummaryHeterophile A‐reactive agglutinins, obtained from the seeds of Dolichos biflorus and from the snails Helix aspersa, Helix pomatia and Cepaea nemoralis, were tested for reactivity with the saliva and red cells of a random series of dogs. Saliva from dogs with the A‐like Tr red cell antigen inhibited the agglutination of human A red cells by each of the reagents. However, none of the heterophile agglutinins distinguished Tr+ from Tr— red cells: the Dolichos agglutinin was non‐reactive, while all of the snail reagents agglutinated both Tr+ and Tr— cells. It is concluded that the canine red cell bears a series of heterophile receptors which mask reactivity with the canine homologue of the red cell A‐antigen, and that the restricted size of the binding sites of the agglutinins limits the informational significance of cross‐reactivity obtained with these reagents.