Summary: Rabbit hemagglutinating antibodies to sulfanilic acid and p-aminobenzoic acid were found to be heterogenous with respect to their molecular size. These differences in the physical properties of the anti-haptene globulins were determined by zone electrophoresis, density gradient ultracentrifugation, ion exchange chromatography and susceptibility to reduction by mercaptoethanol. Examination of the anti-haptene globulins by these techniques indicated that the antibody activity was of both the γ-1, 19 S and γ-2, 7 S types. The proportion of the two types of antibody present in a particular antiserum was found to change with the extent of immunization. After an initial series of antigen injections the antihaptene activity was predominantly the macroglobulin type. With continued immunization the antibody activity shifted to the 7 S type. The distribution of anti-haptene activity between the two globulin fractions changed independently of the physical properties of the antibodies to the carrier protein. Early in the course of immunization the anti-protein globulin was primarily the 7 S type in contrast to the macroglobulin nature of the haptene antibody. These changes in the proportion of 19 S to 7 S antibodies in various antisera do not necessarily represent the same distribution of antibody on a molar basis, since the efficiency of the hemagglutination test in detecting the two antibodies may not be the same. The origin of the two types of antibody is discussed in relation to the inductive phase of antibody formation.