The effects on PGE2 release by lymphocytes obtained from adult thymectomized mice and from normal mice, after incubation with Thymosin alpha 1, were studied. Splenocytes from Tx mice release PGE2 at short time of incubation (15') using 4 micrograms/ml of Thymosin alpha 1 and at longer time of incubation (60'-180' using 4-8 micrograms/ml of Thymosin alpha 1. On the other hand thymocytes release the highest amounts of PGE2 after longer time of incubation (60'-180'). However lymphocytes obtained from normal mice do not release PGE2 amounts comparable with that released by the control samples. This effect shows an interesting interaction between thymic hormones and PGs.