We produced alloimmune sera against erythrocytes of the marsupial, Monodelphis domestica. By absorption fractionation, six operationally monospecific blood typing reagents (hemolysins) were prepared. Using segregation analyses, we found that each reagent detects a blood group antigen controlled by an autosomal dominant gene. With the description of these genetic markers, the utility of M. domestica as an animal model is enhanced.