The effects of photochem. modification of acrylonitrile-vinyl acetate copolymer ultrafiltration membranes by 9 arom. azides on the membrane permeability and dextran selectivity and sensitivity to contact with lysozyme and ovalbumin were examd. 4-Azidobenzoic acid, (4-azidophenyl)trimethylammonium iodide, and Na 4-azidobenzenesulfonate (I) caused increased flux with decreased dextran retention. The other azides showed the reverse effects plus significant flux redn. as a result of ovalbumin contact. The strongest effects were achieved with 2-naphthyl azide and 2-naphthoyl azide. A pronounced pos. effect of photomodification on protein fouling behavior was evident with lysozyme. Using membranes modified with I, a much higher filtrate flux was obsd. compared to the unmodified membrane.