GAT-specific helper T cell lines were shown to bind GAT, coupled to sepharose or sheep erythrocytes. The binding of T cell lines to GAT was specific, since forming of rosettes with GAT-SRBC was inhibited by pretreatment of T cells with free GAT or anti-idiotypic antibodies. Surface iodination of cells and purification of cell lysates on GAT-sepharose, revealed the presence of a 66 KD molecule and a smaller, 33 KD fragment. The proteins could be eluted specifically from GAT-sepharose with a free GAT, or nonspecifically with SDS. The 66 KD molecule could not be reduced by 5% 2-mercaptoethanol. In addition, it was found that the GAT-binding molecule did not bind to the antibodies against the heterodimer (T-cell receptor). The results suggest that the antigen receptor for GAT on helper T cells and inducer suppressor T cells may be related. The GAT-binding molecule may be loosely associated with the heterodimer.