The dependence of grafting and catalytic activity on the pore size of nylon membranes grafted with different DGDA concentrations and loaded with beta-galactosidase is discussed. Membrane pore sizes were 0.2, 1.2, and 3.0 mu m. Grafting and enzyme activity, all other experimental conditions being the same, decrease with the increase of membrane pore size. The membranes have been characterized also with reference to pH and temperature. The experimental conditions giving the best performance for each of the three membranes have been identified. In all cases the membranes giving the best results are those having 0.2 mu m pore size. The advantages of using in nonisothermal bioreactors a single membrane, catalytic and hydrophobic at the same time, are also indicated.