Addition of luminol to the xanthine oxidase system caused the production of chemiluminescence (CL). Superoxide dismutase (SOD) and singlet oxygen (¹O₂) scavengers inhibited the CL, whereas catalase and hydroxyl radical (HO) scavengers had no significant effect, suggesting that ¹O₂ species derived from O^-₂ is responsible for the light emission of luminol. Addition of unsaturated acids to the xanthine oxidase system in the presence of luminol resulted in quenching of CL, and peroxidation of the unsaturated fatty acids was observed. SOD and ¹O₂ scavengers also inhibited the peroxidation of arachidonate, indicating that ¹O₂ generated in the xanthine oxidase system participates in the peroxidation reaction.