Antimicrobial characteristics were investigated for norfloxacin against 18 bacteria (4 gram-positive bacteria and 14 gram-negative bacteria) with with the Minimum inhibition concentration (MIC) test. Among the bacteria tested, B. cereus, V.fluvialis, and V. parahaemolyticus formed norfloxacin-tolerant colonies. The release of ultraviolet-absorbing substances from the cells in contact dwith norfloxacin was examined for the native bacteria and for the norfloxacin-tolerance-induced ones. Norfloxacin was grafted to polypropylene-graft-maleic anhydride (MAPP) and poly-(styrene-co-maleic anhydride) (MAPS) to synthesize polym ric antimicrobial agents. The bioactivity of MAPP-norfloxacin and MAPS-norfloxacin was evaluated and compared with that of neat norfloxacin with the shake flask test and the MIC test. (c) 2005 Wiley Periodicals, Inc.