1. Peptide T and four other aminoacyl derivatives of this octapeptide were synthesized on solid phase support using the Boc and Fmoc procedures. 2. The octapeptides were modified by chloroacetylation and radiolabelled by halogen exchange with 125I. 3. Purified and crude extracts of lymphocytes were used to determine the binding of the octapeptides at different concentrations.