NEW MATERIAL: Tetrapeptide represented by formula I [Z and Z' are each formula II (R1 is phenyl, tolyl, adamantly, cyanoethyl, or the like), formula III (R2 is 2-4C alkyl or the like), formula IV (X is S or O; R3 and R4 are each benzyl, lower alkyl or the like), etc.].

EXAMPLE: N-benzyloxycarbonylsarcosyl-N-trifluoroacethyl-L-lysil-sarcosil-L- glutamineamido.

USE: Intermediates for production of tetrapeptide for improvement of thymus gland function.

PROCESS: For example, the compound of formula I is obtained by allowing N- phenylmethoxycarbonyl-sarcosine-N-hydroxysuccinimide with N-trifluoroacetyl- L-lysine and allowing resulting compound to react with sarcosil-L-glutamineamid in the presence of a condensing agent.