Acid phosphatase from sweet potato was immobilized on the external-solution side surface of a perfluorocarboxylate ion-exchange membrane Two membranes of different enzyme amounts were prepared without any crosslinking reagents The effect of the immobilized-enzyme amount on the enzymatic reaction rate across the membrane was investigated for the hydrolysis of 4-nitrophenyl phosphate The flux of the reaction product. 4-nitrophenol. was found to obey a Michaelis-Menten equation As the amount of the immobilized enzyme increased, the Michaelis constant decreased whereas the maximum flux did not increase