Three new blood group systems, called “T,” “U,” and “V,” have been identified in the rhesus monkey (Macaca mulatta). Each system consists of a single antigenic factor (blood group) detected by a monospecific alloimmune reagent that agglutinates erythrocytes. The antisera that detect these blood groups were obtained following a series of alloimmunizations and absorption fractionizations of the resulting antisera to produce operationally monospecific typing reagents. Analyses of family data indicated that each blood group was controlled by an autosomal dominant gene and that each system was independent of previously defined systems. With the addition of these new blood groups, we can identify 16 different blood group systems and well over one hundred million possible phenotypes in this species.