Antisera were prepared in rabbits against anti-p-azobenzoate antibodies of an A/J and a BALB/c mouse and anti-p-azophenylarsonate antibodies of an A/J mouse. After appropriate absorption the antisera reacted with the anti-hapten antibody of the donor mouse but, by sensitive quantitative tests, not at all with other components of the hyperimmune serum or with preimmune serum of the donor mouse. The absorbed antiserum therefore appeared to be specific for idiotypic determinants. Nearly all idiotypic specificities identified in the serum of the donor were also present in the serum of other mice of the same strain, immunized against the same hapten group, but not in mice immunized with a different hapten. In each case the antibodies of the donor mouse reacted most effectively on a weight basis with antiidiotypic antiserum. Cross-reactions were observed among different strains of mice but homologous anti-bodies reacted most effectively with antiidiotypic antisera. C57/BL and DBA antisera contained very low concentrations of specificities present in the A/J and BALB/c antibody populations; antibodies of A/J and BALB/c antisera are more closely related to one another. The results indicate that idiotypic specificity may provide a genetic marker for the variable regions of immunoglobulin polypeptide chains.

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