A fragment analog, [D-Arg30]prothymosin alpha fragment 1-30, containing D-arginine in place of arginine residue at position 30 was synthesized by the liquid phase procedure and studied for immunological effect on the impaired blastogenic response of T-lymphocytes isolated from uremic patients after treatment of human serum. Deacetyl-thymosin alpha 1, a synthetic octaeicosapeptide corresponding to deacetyl-prothymosin alpha fragment 1-28, has restoration ability for the impaired blastogenic response of T-lymphocytes of uremic patients but is susceptible to proteolytic digestion. On the other hand, the fragment analog, [D-Arg30]prothymosin alpha fragment 1-30 retained activity and was shown to exhibit a high degree of stability when incubated in human serum. These results indicate that N-terminal acetylation and the introduction of D-residue into the C-terminal residue of prothymosin alpha fragment 1-30 increase resistance to proteolytic degradation by exopeptidases.