The synthetic terpolymer of L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT) induced anti-GAT antibodies in D1.LP mice. Some of these antibodies shared common idiotypic specificities. The interaction between idiotypic antibody and anti-idiotypic antiserum was specifically inhibited by GAT and the closely related polymer of L-glutamic acid and L-tyrosine (GT) but not by the copolymer of L-glutamic acid and L-alanine (GA). The binding of labeled GAT by the anti-GAT antibodies was also specifically inhibited by the anti-idiotypic antiserum. These results indicated a close association of idiotypic determinants with the antibody-combining sites. Furthermore, such active site-related common idiotypic specificities were found in individual sera of both responder mice (D1.LP) and nonresponder mice (DBA/1) immunized with GAT and GAt-MBSA, respectively, indicating that both responder and nonresponder mice carry B cells with common clonotypes.