The multichain co‐polymer poly(Tyr,Glu)‐polyAla‐polyLys, designated (T,G)‐A‐L, is normally thought to present only the poly(Tyr,Glu) sequences, designated (T,G), as antigenic determinants. Evidence is presented indicating that at least two different determinant in the (T,G)‐A‐L antigen arc recognized by chicken. Studies in a partly inbred high‐responder chicken strain reveal two major determinant systems: the (T,G) and the A‐ ‐I. in the (T,G)‐A‐ ‐I. antigen. For one serum the antigenic determinant of the A‐ ‐I. backbone is