Structure-activity relationship of antioxidants for the protective effects on linoleic acid hydroperoxide (LOOH)-induced toxicity were examined in cultured human umbilical vein endothelial cells. alpha-Tocopherol, 2,2,5,7,8-pentamethylchroman-6-ol, butylated hydroxytoluene, probucol, and fatty acid esters of ascorbic acid provided efficient protection against the cytotoxicity of LOOH in pretreatment, but phenols without alkyl groups at the ortho positions and hydrophilic antioxidants such as Trolox and ascorbic acid provided no protection. Probably, the effectiveness of the protection against cytotoxicity by these antioxidants dependsprimarily on their rate of incorporation into cells due to their lipophilicity, secondly on their antioxidant activity, and thirdly on their orientation in biomembranes. On the other hand, flavones, such as baicalein and luteolin bearing 3 to 5 hydroxyl groups, and flavonols showed a protective effect against LOOH cytotoxicity when added together with LOOH,but not by pretreatment. Among catechins, (+)-catechin and (-)-epigallocatechin gallate monoglucoside and diglucoside were effective in suppressing LOOH-induced cytotoxicity, but their effects were not so strong. The structure-activity relationship of flavonoids revealed the presence of either theortho-dihydroxy structure in the B ring of flavonoids or the 3-hydroxyl and 4-oxo groups in the C ring to be important forthe protective activities. Furthermore, coumarins such as esculetin containing the ortho catechol structure had protective effects in both pretreatment and concurrent treatment. These results suggest that ortho catechol moiety of flavonoids, catechins, and coumarins is an important structure in the protection against LOOH-induced cytotoxicity,and that the alkyl groups of monophenols are critical for protection.