Agglutinating substances having characteristics of naturally occurring macroglobulin antibodies to human Bence Jones proteins have been identified in human sera. By means of hemagglutination and hemagglutination inhibition techniques, common determinants have been demonstrated on the light (L) polypeptide chains of pooled normal human gamma(2)-globulin and on some Bence Jones proteins of group 1 but not of group 2. Individual human sera serve to delineate subgroups of the two major antigenic groups of the Bence Jones proteins by agglutinating cells coated by one but not another protein of the same antigenic group. The complexity of subgroups, especially of group 2, is established by testing a panel of Bence Jones proteins of the same group for their ability to inhibit hemagglutination. By this means it appeared that different sera recognized different group-specific determinants of cells coated with a single Bence Jones protein. The capacity of the L polypeptide chains and proteolytic fragments of gamma(2)-globulin to inhibit the hemagglutination reaction between Bence Jones protein or L chain-coated cells and human sera was examined. These studies demonstrated that the determinants, toward which agglutinators of human serum are directed, appear to be blocked in intact gamma(2)-globulin and in all fragments in which H chain remains in proximity to L chain. It would appear that the presence of H chains bound to L chains by non-covalent bonds completely obstructs the reactivity of the involved L chain groups. The agglutinating capacity of a serum toward Bence Jones proteins or L chains of gamma(2)-globulin appeared to be independent of its agglutinating capacity for cells coated with intact gamma(2)-globulin. No correlation of the presence in serum of agglutinators for Bence Jones proteins or L chains with health or disease has been established.