Chemoprevention with food phytochemicals is currently regarded as one of the most attractive strategies for cancer control. We have been continuously working on the identification and characterization of chemopreventive phytochemicals extracted from a diverse range of edible plants. Recently, we have utilized a convenient assay, the tumor promoter-induced superoxide generation test, with differentiated HL-60 cells for primary screening, and performed chemical studies of antioxidative food phytochemicals. Here, we report our criteria for evaluation of new types of chemopreventors on the basis of the involvement of leukocyte-derived reactive oxygen in carcinogenesis.