A marked homogeneity of the light chains was observed in an analysis of 17 IgM proteins with anti-γ-globulin activity. The V region subgroups of the light chains were determined by both sequence and antigenic analysis. The latter procedure permitted large scale screening for comparisons with control proteins. The two methods showed general agreement in the determination of Kappa III proteins; all proteins positive by antigenic analysis were also positive by sequence but exceptions were noted in the opposite direction. The anti-γ-globulins showed by antigenic analysis a 92% incidence of VK III light chains as compared to an incidence of 27% among 81 control proteins without this activity. A similar selection was observed for an antigen (VK III b) which subdivided the kappa III proteins.
The major Wa group of anti-γ-globulins which had been delineated previously on the basis of cross-idiotypic specificity was primarily responsible for the special light-chain selection. All the proteins of this group contain VK III light chains and all were of the VK III b type. Current evidence indicates that additional light-chain similarities were involved in the cross-idiotypic specificity of the Wa group. It thus appears that for the anti-γ-globulins various levels of selection of light chains are manifest ranging from a preponderance of kappa type, of kappa III subgroup, of kappa III b sub-subgroup and perhaps of still further subdivisions to account for the cross-idiotypic specificity.