A preliminary study was made on the feasibility of using papain as a catalyst in the synthesis of aspartame precursor (Z-Asp-OH + Phe-OMe-->Z-Asp-Phe-OMe). The synthetic reaction was carried out by an equilibrium-controlled process using three different systems : precipitation, dissolved-state, and biphasic. A comparative study was made with thermolysin-catalyzed aspartame precursor synthesis in each reaction system. Papain was useful only when the biphasic system was employed for synthesis, while thermolysin was efficient in all the systems tested. Thermolysin appeared to be superior to papain even in the biphasic system; the reason for this was ascribed to the absence of esterase activity. The yield of Z-Asp-Phe-OMe in the biphasic system was increased when papain was used in a reaction mixture containing high concentrations of the starting materials. The likely reason for this is discussed.