Trypsin-catalyzed, kinetically controlled synthesis of a precursor, dipeptide of thymopentin (TP-5), Bz-Arg-Lys-OH (or-OEt) in organic solvents was studied. Bz-Arg-OEt was used as the acyl donor and Lys-OH and Lys-OEt were used as the nucleophiles. Ethanol was selected as the organic solvent from ethanol, methanol, acetonitrile, and ethyl acetate tested under the experimental conditions. As expected, Lys-OEt is not a suitable nucleophile in trypsin-catalyzed reaction, due to its competition with the protective Arg-OEt as acyl donor for the active site of trypsin, while Lys-OH does not have this problem. The optimal reaction condition for the synthesis of Bz-Arg-Lys-OH was set up as 20% Tris-HCl buffer, pH 8.0, 35 degrees C for 6 h with the yield of 52.5%, or for 18-24 h with the yield of about 60%.