From BALB/c mice immunized with BALB/c polyclonal anti-GAT antibodies, we have obtained monoclonal antiidiotypic antibodies directed against public idiotopes expressed following GAT (Glu60Ala30Tyr10) immunization in all individuals of all mouse strains tested. Nine monoclonal anti-Id antibodies were studied in detail. One of these monoclonals recognizes only polyclonal anti-GAT antibodies; the other eight recognize polyclonal anti-GAT antibodies and monoclonal anti-GAT antibodies. The distribution of the structures recognized by the different monoclonal anti-Id on a battery of twenty monoclonal anti-GAT antibodies allows a definition of two families of public idiotopes. The significance of the existence of these two families is discussed in light of the hypothesis recently proposed by Jerne et al. (EMBO J., 1982, 1, 243-247) who consider that the induction of internal images of antigen during immunization leading to the production of antiidiotypic reagents is responsible for the existence of public idiotopes. Our present results may be interpreted to indicate that even though certain monoclonal anti-Id reagents could be internal images of the antigen, the others do, in fact, recognize public idiotopes. Therefore, the notion of internal image does not exclude the existence of public idiotypic specificities.