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Further delineation of the antigens characteristic of the Vi or gamma(2c) subgroup of gamma-globulin was carried out utilizing a number of rabbit and primate antisera. Two genetic antigens characteristic of this subgroup, Gm(b) and Gm(g), were also detected by precipitation techniques with certain of the antisera. These were clearly differentiated from antigens common to all proteins of this subgroup. The concentration, of Vi protein in normal and pathological sera from several population groups was measured quantitatively utilizing a variety of immunological procedures. All sera studied showed measurable levels. The mean value for Caucasian sera was 1.06 mg/ml, representing approximately 8% of gammaG-globulin. This agreed closely with a figure of 8.4% for the incidence of myeloma proteins of the Vi subgroup among all gammaG-myeloma proteins in Caucasians. A relationship was found between the Vi subgroup concentration and the specific genetic type of a given individual. Measurements of the Gm(b) genetic determinants, which are found solely in Vi-type proteins, brought forward this relationship. Gm(b+) individuals showed higher concentrations of Vi-type gamma-globulin than those who were Gm(b-), and this difference was statistically significant for both the homozygous and heterozygous states. It appeared that the structural genes for Gm(b+) polypeptide chains showed a greater synthetic capacity than those for Gm(b-) types. The possible significance of such effects in governing the relative composition of the antibody population in a given individual is discussed.