A composite microporous membrane made of a functional polymer that showed remarkable affinity with viruses was developed as a filter material for removing viruses from air. A 0.83-mm thick membrane with pore diameter of 14.3 mu m and porosity of 42% was prepared that consisted of a large number of connected minute beads of 1.7 mu m in diameter made of crosslinked poly(N-benzyl-4-vinyIpyridinium chloride) and reinforced by nonwoven cloth. Filtration experiments were performed using bacteriophage T4 as a test virus.When influent concentration was in the order of 106 PFU/ml, filtration using one sheet of the membrane showed 99.9994-99.9998% removal (5.2-5.7 log(10)-unit reduction in concentration) of the virus from air. Percentage of removal increased with decrease of influent concentration, and the virus was not detected in the filtrate when influent concentration was in the order of 10(5) PFU/ml. Since pore of the membrane was greatly larger than the virus, the excellent removal can be attributed to the special and remarkable affinity of the pyridinium-type polymer with viruses.