In the theory of hemolytic transfusion reactions and of hemolytic disease in the fetus and the newborn, theRh0factor holds a special position. It is the most antigenic of the Rh-Hr blood factors and the usual source of clinical complications. Because of the recent discovery of new antibodies within this system it has become necessary to assume that standard Rh-positive blood contains factorsRhA,RhB,Rhc, and others, as well asRh0. The theory and nomenclature here developed makes it easier to understand the interrelations of these factors and should aid the physician in solving certain puzzling clinical problems. Its predictive value makes it helpful in planning further investigations.