The purpose of the present work has been to seek a correlation of potential predictive value, demonstrating redox control of cytotoxicity toward human nasopharynx carcinoma (KB) cells by seven 5-hydroxy-7-methoxyflavones, together with two glycoside derivatives, all extracted from Lethedon tannaensis. In this approach, redox control is characterized by a physicochemical parameter expressing quantitatively the relative electron-donating power of the flavones, this parameter being the second order rate constant, kdelta, for quenching of singlet oxygen O2 (1deltag). This rate constant kdelta is usually related to the ability of a given molecule D to donate an electron and, thus, with the reduction potential E of the couple (D*+/D). Our results show that the flavone toxicity is linearly correlated with ease of oxidation: the higher the rate constant of reaction with singlet oxygen, the easier the oxidation, the less positive or more negative the reduction potential ((D*+/D), the higher the cytotoxicity. The results suggest new screening strategies to identify and improve potential antitumor drugs.