Copper chloride was found to be an extremely efficient additive suppressing racemization in the carbodiimide mediated couplings. The use of additives in the dicyclohexylcarbodiimide (DCC) method is an important improvement contributing to the suppression of racemization in segment couplings. Among a variety of N-hydroxy compounds proposed as such additives, 1-hydroxybenzotriazole (HOBt), for example, proved to be very effective in many peptide models and is preferably used in the practical synthesis of long peptide chains. Unfortunately, however, even the addition of this reagent cannot always guarantee couplings against racemization. 1) Thus additives preventing racemization completely are eagerly desired. Recently we have investigated the factors which influence the degree of racemization in the model couplings of a series of benzyloxycarbonylglycylamino acids with amino acid esters, taking advantage of the good separation of diastereomers and high accuracy of analysis by reversed phase high performance liquid chromatography (RP-HPLC). During the course of this investigation we have evaluated the racemization suppressing effect of a number of additives in the carbodiimide method and have found that copper(I1) chloride is an extremely efficient and promising additive as compared with the hitherto known ones. Concerning the use of such in-