During investigation of the isoantibodies produced by immunizing transfusions in a series of randomly cross-transfused dogs, a variety of blood grouping systems analogous to human ones were discovered, and it is believed that others exist which could be identified by further systematic study. The discussion touches upon the incidence of blood group antigens in normal dogs, characteristics of the isoantibody systems, the life span of red cells, transfusion reactions, organ transplantation experiments, and hemolytic disease of newborn dogs-topics of theoretical biological interest and also of practical importance in veterinary medicine and in experimental studies using dogs.