AbstractÐA series of zeolite immobilized a-chymotrypsin and thermolysin with microporous Y zeolites (HY, NH 4 Y, NaY) and mesoporous dealuminized DAY zeolites (HDAY, HNH 4 DAY) as matrixes have been prepared to catalyze peptide bond formation in organic solvents for the ®rst time. The results indicated that most zeolite immobilized enzymes were active for peptide syntheses in organic media, and still had catalytic activity to some extent after being reused ®ve times. According to the results, the immobilization effect of microporous Y zeolite was better than that of mesoporous DAY zeolite, suggesting that microporous Y zeolite can form more powerful hydrogen bonds with enzyme molecules since there are more hydroxyl groups on the Y zeolite than on the DAY zeolite. In addition, the in¯uences of some reaction conditions such as reaction time and water content of the solvent on the enzymatic peptide synthesis were also studied and optimized. For the two kinds of proteases, NH 4 Y zeolite did not show its advantages for thermolysin, but was more suitable for a-chymotrypsin as an immobilization matrix.