Methyl methacrylate (MMA) was grafted onto a cellulose triacetate microfiltration (MF) membrane by radiation-induced graft polymerization (RIGP). Radicals were produced in the cellulose triacetate MF membrane after the irradiation with electron beams. The trapped radicals were then reacted with the MMA monomer both in vapor and liquid phases. In the liquid-phase grafting, the slower rate of graft polymerization was due to the occurrence of a chain transfer of radicals in the membrane. The grafted polymer branches were satisfactorily isolated from the graft copolymer by means of an acid hydrolysis method. From the gel permeation chromatogram, the graft chains prepared by vapor-phase grafting had a higher number-average molecular weight than those obtained by liquid-phase grafting at an identical degree of grafting. The length of the poly-MMA chain grafted in a vapor phase onto the cellulose triacetate MF membrane ranged from 10(3) to 10(4) monomer units and the degree of grafting ranged from 50 to 250%; its extended length was tantamount to the pore radius of the starting MF membrane.