A novel cell-containing assay system using electron spin resonance (ESR) and spin-trapping methods was established. We used this assay system to evaluate the antioxidant activities of various dietary antioxidants including resveratrol, lycopene, quercetin and rutin. Resveratrol showed the strongest radical-scavenging activity among the compounds tested. Our results provided useful data for the establishment of a model for evaluating the antioxidant activities of natural dietary substances in cell-containing systems.