SummaryAnti-I, anti-i, anti-H and anti-HI sera of human origin were used to examine the erythrocytes of rabbits, monkeys, and guinea pigs for the presence of I, i, H, and HI blood substances. In direct agglutination experiments with the four antisera after they had been absorbed with rabbit, monkey, guinea pig, human group OI or human group Oi (cord) red cells, it was evident that the erythrocytes of rabbits possessed I, H, and HI but no i substances. Monkey red cells had i but no I, H, and HI blood group activity. Guinea pigs were shown to have I, H, and HI, antigens but the i-status of their erythrocytes remained equivocal. Further absorption elution experiments revealed unequivocally that guinea pig red cells possessed the i blood group antigen.The authors thank Marie C. Crookston, Division of Haematology, Toronto General Hospital, Toronto, Ontario, Canada, for her generous gift of the anti-i serum (Hog).