The heterogeneity of the anti-GAT [terpolymer poly(Glu60, Ala30, Tyr10)] response of GAT responder mice has been analyzed. Purified anti-GAT antibodies from BALB/c mice belong only to the gamma 1 kappa subclass. The isoelectric focusing pattern obtained indicates that the anti-GAT antibodies are particularly basic and restricted. These results have been confirmed by two-dimensional polyacrylamide gel electrophoresis; by this technique, we have shown that the gamma 1 chain and the corresponding kappa chain of anti-GAT antibodies are restricted. All the anti-GAT antibodies from 10 BALB/c mice bear the cross-reactive GAT idiotype previously defined (J. Thèze and G. Sommé, Eur. J. Immunol. 1979, 9: 924). The present results indicate that the anti-GAT repertoire expressed in BALB/c mice is very limited.