Antibodies to the synthetic polypeptide (T,G)-A--L were raised in C57BL/10 and C3H.SW mice. For each strain, the anti-(T,G)-A--L antibodies from 10 animals were pooled, affinity purified on a (T,G)-A--L-Sepharose column, and used to immunize Lewis rats. The resulting rat antisera were adsorbed with insolubilized normal mouse globulin in order to remove anti-isotypic and antiallotypic antibodies. The residual antibodies specifically inhibited the binding of (T,G)-A--L by anti-(T,G)-A--L as measured by a radioimmunoassay. The specificity of this inhibition was demonstrated as follows: 1) failure of the anti-(T,G)-A--L anti-idiotype to inhibit the binding of nuclease to anti-nuclease antibody of the same allotype; 2) failure of Lewis anti-[B10 anti-(T,G)-A--L] to inhibit C3H.SW anti-(T,G)-A--L and vice versa; 3) ability to absorb anti-C3H.SW anti-idiotypic activity on insolubilized C3H.SW anti-(T,G)-A--L but not on B10 anti-(T,G)-A--L. The same or cross-reactive idiotype(s) was present in the majority of individuals of each of these strains.