Fifteen hybridoma anti-poly(Glu60Ala30Tyr10) (anti-GAT) antibodies were analyzed for the presence of a common set of idiotypic specificities associated with murine anti-GAT antibodies, termed CGAT idiotype, which are present on the anti-GAT antibodies of all mouse strains. Thirteen of these monoclonal anti-GAT antibodies expressed a major fraction of CGAT idiotypic specificities. However, the remaining fraction of CGAT idiotypic specificities were not detected in individual or pooled hybridoma anti-GAT antibodies. Anti-idiotypic antisera made against each of the 15 hybridoma anti-GAT antibodies preferentially bound homologous ligand and showed minimal binding activity to specifically purified serum anti-GAT antibodies. Furthermore, the diversity of the hybridoma anti-GAT antibodies was demonstrated by the presence of individual idiotypic specificities on each of the hybridoma anti-GAT antibodies. However, relatedness among some of the hybridoma antibodies was also apparent since idiotypic analysis revealed that some hybridoma anti-GAT antibodies shared cross-reactive idiotypic specificities not associated with CGAT idiotype. The genetic mechanisms which could account for the generation of such antibody diversity are discussed.