Incubation with Thymosin Fraction 5 induces a dose-dependent release of PGE2 by lymphocytes obtained from adult thymectomized mice; Indomethacin inhibits this effect. The same result was not observed in lymphocytes obtained from normal mice. PGE2 release is correlated with Thy-1 appearance on Thy-1 negative lymphocytes, after incubation with Thymosin, valutated with Bach's rosette inhibition test.