Summary: Marked differences in antibody-activity are found in human γ1-and γ2-globulin antibody preparations. The hemagglutinins and typhoid “O” agglutinin appear to be γ1-globulins. The β-hemagglutinin of plasma represents a small amount of the high molecular weight portion of the γ1-globulin. The antibody responses of rabbits to human γ1- and γ2-globulins, fibrinogen and albumin as studied by the antigen-dilution technic showed that these essentially electrophoretically pure proteins did not represent distinct immunological entities.