Cyclic N-halamine biocidal monomers and polymers and methods of using the same as biocides, wherein the functional group, halogenated oxazolidinones, may be homo- and copolymerized, are provided. The copolymerizations are effected with inexpensive monomers such as acrylonitrile, styrene, vinyl acetate, vinyl chloride, and the like. Grafting reactions were also accomplished with the N-halamine monomers and commercial polymers such as poly-acrylonitrile, poly-styrene, poly-vinyl acetate, poly-vinyl alcohol, poly-vinyl chloride, and cellulose. These N-halamine compounds are stable biocides which release small amounts of free halogen and other impurities. They will be useful as disinfectants for swimming pools, oil and water based paints, preservatives, medical and dental coatings, industrial and commercial coatings, fabrics, sterile bandages, liners of containers, and the like.