There is an increasing evidence that oxidative stress is implicated in the processes of inflammation and carcinogenesis. It has been shown that kahweol and cafestol, coffee-specific diterpenes, exhibit chemoprotective effects. This study investigated the effects of kahweol and cafestol, coffee-specific diterpenes, on the hydrogen peroxide (H(2)O(2))-induced oxidative stress and DNA damage in NIH3T3 cells. When the cells were treated with kahweol or cafestol, cytotoxicity, lipid peroxidation, and reactive oxygen species production induced by H(2)O(2) were markedly reduced in a dose-dependent manner. Moreover, kahweol and cafestol were shown to be highly protected against H(2)O(2)-induced oxidative DNA damage as determined by the Comet (single cell gel electrophoresis) assay and the measurement of 8-oxoguanine content in NIH3T3 cells. Kahweol and cafestol also protected hydroxyl radical-induced 2-deoxy-d-ribose degradation by ferric ion-nitrilotriacetic acid and H(2)O(2). In addition, kahweol and cafestol efficiently removed the superoxide anion generated from the xanthine/xanthine oxidase system. These results suggest that kahweol and cafestol are effective in protecting against H(2)O(2)-induced oxidative stress and DNA damage, probably via scavenging free oxygen radicals, and that kahweol and cafestol act as antioxidants.