Insoluble crosslinked polystyrenes (PS) grafted with different quaternary ammonium groups were synthesized as antibacterial agents. Their antibacterial activities against Staphylococcus aureus were investigated by a colony count method. It was found that when the concentration of the insoluble antibacterial agent was low, the antibacterial activity was different from that of the typical soluble antibacterial agent. The antibacterial activity was bacteriostatic rather than bactericidal. A model for explanation of the antibacterial activity of the insoluble ammonium salt was set up. (c) 2005 Wiley Periodicals, Inc.