Two analogs of thymosin β₄ the N-terminal acetyl groups of which were substituted by Ala or D-Ala, were synthesized by the solution method and studied for their immunological effect on the impaired blastogenic response of T-lymphocytes isolated from uremic patients after treatment of human serum. One of the synthetic analogs, D-Ala-thymosin β₄ demonstrated a restorative effect on these patients when incubated in human serum, but the other analog, Ala-thymosin β₄, showed no restorative effect under the same conditions. These results seem to suggest that D-Ala-thymosin β₄ increases resistance to proteolytic degradation by exopeptidases more than Ala-thymosin β₄.