In the U. S. S. R. the therapeutic use of blood-transfusion in horses has proved to be economically important, and this led us to a study of the isoagglutinative properties of horse-blood. A preliminary study of the blood of 18 horses indicated the presence of 4 groups, corresponding to those in man. We designated them by numbers, as in Jansky's system, but of course the choice of designations for groups II and III was arbitrary. Examination of sera sent us by Prof. N. I. Sniguirov, of the Zoötechnical-Veterinary Institute of Saratov, who among others has studied equine blood-groups, showed that he had made just the opposite choice, and his group II corresponded to our group III and vice versa. This, together with the possible identity of our groups II and III with the groups “A” and “B” of Hirszfeld and Prezemysci, Thomoff, and others, led us to seek a more definite basis for our nomenclature.