A wide assortment of antibodies has been analysed by qualitative and quantitative techniques for their content of group I- and group II-type gamma-globulin molecules. These included Rh antibodies, isoagglutinins, thyroglobulin antibodies, dextran antibodies, and teichoic acid antibodies. All the antibodies studied showed the presence of both groups. However, the ratio of the two varied widely in different individuals and diverged markedly from the ratio in the total gamma-globulin of the individual. Two antibodies isolated from the serum of the same individual showed different ratios for the two groups.