An immune serum produced by injecting human urinary A-enzyme preparation to blood group O' rabbit had an ability to inhibit both A-enzyme actions of human and rabbit, and an immune serum produced by injecting the same A-enzyme preparation to blood group A' rabbit only inhibited the human A-enzyme action. The two immune sera also had an ability to inhibit the human B-enzyme action. Besides the specific anti-B-enzyme antibody, similar cross-reacting antibody was produced in the serum of blood group A' rabbit which was immunized with human urinary B-enzyme preparation.