Summary: The antibodies, produced in rabbits against human erythrocytes, of the primary and secondary response have been studied with reference to the possibility of their being qualitatively different. In agglutinating activity no difference was detected and no “incomplete” agglutinin was demonstrated. However, primary and secondary response antibody appeared to differ in their hemolytic activity in that primary response sera were more weakly hemolytic with rabbit complement and with low concentrations of guinea pig complement than were secondary response sera, even when the hemagglutinin content was comparable.