The polypeptide antigen poly-L-(Tyr,Glu)-poly-D,L- (Ala)-poly-L-(Lys), designated (T,G)-A-L, is under the control of H-2 linked Ir genes. Antibodies to (T,G)-A-L are of restricted heterogeneity, recognizing two distinct sets of epitopes. A significant proportion of antibodies bind to a (Glu,Tyr)-containing (GT) structure that is also found on linear polypeptides including poly-L-Glu60- Ala30-Tyr10 (GAT).