New analogs of the serum thymic factor, (Phe-NH210)-FTS, (Gln1, Phe-NH210)-FTS, and (des Glp1, Phe-NH210)-FTS were synthesized. Their biological activities were evaluated in the recovery assay of T lymphocytes. The activity of (Phe-NH210)-FTS is comparable to the activity of FTS. Nonapeptide (des Glp1, Phe-NH210)-FTS shows the highest activity so far observed. Analog (Gln1, Phe-NH210)-FTS is an inhibitor.