PURPOSE: To prepare 6-aminopenicillanic acid efficiently from penicillin by using an immobilized enzyme obtained by immobilizing penicillinacylase to a specific porous weakly basic anion exchange resin.

CONSTITUTION: A porous weakly basic anion exchange resin comprising a styrene/divinylbenzene copolymer as a parent nucleus and an introduced polyamine as an exchange group, having about 200W1,500 pore diameter distribution and about 100W1,000μ particle diameters, is used as a carrier, penicillinacylase, preferably penicillinacylase prepared by a bacterium such as Bacillus megatherium, is immobilized on a carrier using a diisocyanate or diisothiocyanate as an activator, the prepared immobilized enzyme is brought into contact with an aqueous solution of penicillin, and formed 6-aminopenicillanic acid is collected.