Most anti-GAT antibodies in the BALB/c strain express a public idiotypic specificity (pGAT), which is encoded by specific germline genes (VH10, VK5.1 and VK1A5). One or both of these germline genes, referred to as "GAT-specific genes", are also used by four anti-alprenolol antibodies. Anti-Alp and anti-GAT antibodies show no cross-reactivity for the antigens. The light chain of one anti-Alp antibody, 22C4, is encoded as the anti-GAT antibodies by a VK5.1-J2 combination and expresses part of the pGAT idiotopes, whereas the heavy chain is not "GAT"-related. Two anti-Alp using the VH10-VK5.1-J1 association do not express any of the pGAT idiotopes. Sequence comparison of the various CDR sequences points to the predominant role of the VH-CDR2 and VL-CDR3 for the constitution of the pGAT specificity. Regarding VL-CDR3, a drastic change in idiotypic determinants appears to be linked to V-J junctional diversity.