The octapeptide corresponding to human interferon‐α2 (Hu IFN‐α2) sequence 131–138 hu??? high affinity to murine thymocyte receptors (K
6 = 4.2 × 10−11 M, about 700 receptors per cell). The peptide/receptor binding is inhibited by both Hu rl FN‐α1(K
1 = 8.6 × 10−10 M) and thymosin‐α1 (TM‐α1) (K
1 = 3 × 10−??? M) as well as by the octapeptide homologous to the TMα‐α1 sequence 16–23 (K
1 = 4.5 × 10−1 M). The peptide from IFN‐α1 (131–138) activates murine thymocyte blast transformation at a concentration of 10−11 M in the presence of 2.5 μ/ml of concanavalin A.