Anti-idiotypic immunization triggers the production of antibodies that are structurally related to the idiotype. We have shown that the heavy chain variable regions of antibodies produced after anti-ABPC48 (A48) anti-idiotypic immunization of BALB/c mice are homologous to that of A48, except for the third hypervariable region. We present here partial light chain sequences of A48 and of antibodies induced by anti-idiotypic immunization. Nearly perfect homology is found, suggesting that these chains are the products of genes derived from a unique VK germ-line gene. These observations indicate that the H and L hypervariable regions contribute to define the structure of A48 idiotopes. Remarkably, the VK sequence we identify is the same as that described for anti-arsonate and anti-oxazolone antibodies. We discuss the relative importance of particular amino acids for idiotype expression and antigen-binding activity.