Enzymatic peptide synthesis was investigated using carboxypeptidase Y immobilized with glutaraldehyde on 10 pm microparticulate amino-silica. Carboxypeptidase Y was immobilized with 98.5% recovery of active enzyme to yield the immobilized enzyme having 0.55 units esterase activity/mg amino-silica support. The stability of the immobilized enzyme was examined as a function of pH, temperature, and reactant concentrations. Immobilized carboxypeptidase Y was used in stirred batch and recirculating packed-bed reactors for peptide synt h e s i s . Packed-bed r e a c t o r s ( 4 0 x 4.6 m m , 60 x 4.6 mm) were used to catalyze the synthesis of 170 mg N-benzoyl-L-arginyl-L-methioninamide, 380 mg Nbenzoyl-L-arginyl-L-methionyl-L-leucinamide, and 200 mg N-benzoyl-L-arginyl-L-methionyl-L-leucyl-Lphenylalaninamide in 8, 3, and 1 hour, respectively, as intermediates in the synthesis of L-methionyl-L-leucyl-Lphenylalanine. No inactivation of the immobilized enzyme was observed during the course of the reactions. The N-benzoyl-L-arginyl group served to increase the water solubility of the peptides and was removed by immobilized trypsin at the end of synthesis to obtain the final product. While t h e first two s y n t h e s e s were conducted with aqueous reaction mixtures, the synthesis of N-benzoyl-L-arginyl-L-methionyl-L-leucyl-L-phenylalaninamide was carried out in a reaction mixture containing dimethylformamide to avoid precipitation of the product. HPLC and amino acid analysis confirmed the high purity and amino acid composition of the final product.