THE occurrence of cross idiotypic specificity among human red cell cold agglutinins is well known^1–3, and there is evidence that the antigen binding site is involved in these idiotypic determinants. Two distinct idiotypic systems have been described which correlate with the combining specificities of these antibodies. The first is found among IgM cold agglutinins with blood group I and i specificities and the second among proteins of IgM and IgA class with blood group Pr specificities². The idiotypic determinants in each system are numerous and variable numbers are expressed on proteins from different individuals^2–4. Sensitive binding assays showed one idiotypic determinant to be common to at least 73% of cold agglutinins with specificity for the I–i antigen complex³. With the antisera raised against the intact proteins the relative roles of the heavy and light chains in the cross-specificity determinants could not be ascertained because both chains in combination were required for their expression². In the studies reported here, rabbit antisera against the isolated heavy chains of an anti-I and an anti-i cold agglutinin have been used and cross idiotypic specificity among the heavy chains of this group of antibodies has been demonstrated.