At least one acryloyl monomer is polymerized in an aqueous suspension containing a penicillin amidase-producing microorganism. The acryloyl monomers employed in the present invention include acryloylamide, N,N'-lower alkylene-bis-acryloylamide, bis(acryloylamidomethyl)ether and N,N'-di-acryloylethyleneurea. The resultant immobilized penicillin amidase-producing microorganism is subjected to enzymatic reaction with penicillin. 6-Amino-penicillanic acid is produced.