AbstractPolymeric membranes are increasingly used as supports for the immobilization of enzymes in bioreactors. One of the more common reactor types employed in lipase‐catalyzed hydrolysis of oils, contains modified cellulose as a membrane material. We found that this type of material is readily attacked by cellulase present in several commercially available lipase preparations. This leads to membrane damage, reactor instability, and leakage. We conclude that cellulose membranes are not suitable as supports in bioreactors for the immobilizartion of these lipases. The development of alternative membranes is currently in progress. © 1992 John Wiley & Sons, Inc.