A wide variety of polyphenolic compounds (i.e., 4-methylcoumarins, xanthones, pyrazoles, and pyrazolylacrylonitriles) have been examined for their antioxidant effect on NADPH-catalyzed liver-microsomal lipid peroxidation with a view to establishing their structure–activity relationship. Dihydroxy-/diacetoxy-4-methylcoumarin derivatives have been shown to be radical scavengers. The effect of nine different xanthones has been examined on the modulation of cytokine-induced expression of intercellular adhesion molecule-1 (ICAM-1) in human endothelial cells. 1,4-Dihydroxyxanthone showed enhanced antioxidant activity as well as ICAM-1 inhibitory activity.