A study was done of the ultrafiltration properties of expanded polytetrafluoroethylene (ePTFE) films with two average pore sizes, 0.5 and 3.0 mum, grafted with 2-(dimethylamino)ethyl methacrylate (DMAEMA)-grafted 0.5-ePTFE and grafted 3.0-ePTFE-using dextran (Dxt) and quaternized dextran (qDxt) in water and in buffer solutions with a pH in the range of 4-10. The water permeability was found to be proportional to the operating pressure below 2.0 kgf/cm(2). Grafted 0.5-ePTFE films apparently had a higher rejection than did the grafted 3.0-ePTFE films, and the cutoff value for grafted 0.5-ePTFE films decreased with an increase in the grafted amount. The apparent rejection of Dxt and qDxt increased with a decrease in the pH value, and the apparent rejection of qDxt was higher than that of Dxt when the pH was in the range of 4-8 because of an electrostatically repulsive interaction between positively charged grafted PDMAEMA chains and qDxt molecules. For grafted 0.5-ePTFE films, 40KDxt was selectively separated from the 40KDxt/250KDxt mixture systems in water and from the 40Kxt/40KqDxt mixture systems at pH 6 using the difference in their apparent rejection; the separation factors in both systems increased with the grafted amount. These results indicate that grafted ePTFE films are applicable to positively charged ultrafiltration membranes.