The existence of two qualitatively different subtypes in Group II bloods is confirmed with the method of cold agglutination. Two cold agglutinins are shown to be present in Group II sera, one of these acting on bloods AA1, the other on bloods AA2. Group I bloods are characterized by the presence of a structure which is not common to all bloods, but which seems to be similar to that of A2. Some observations made with cold agglutinins would indicate that there exist a number of individual

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... ces in human bloods, the phenomena being much less pronounced than typical isoagglutination. From several known facts it seems probable that the individual differences such as have been demonstrated by serological reactions are due to variations in substances other than proteins.