Natural triterpenoids such as ursolic and oleanolic acids have been detected to suppress enzymes which play a role in liver damage, e.g. cytochrome P450, cytochrome b5, CYP1A and CYP2A, and to increase the antioxidant substances glutathione, metallothioneins, and glutathione-S-transferase, with simultaneous protective effects on liver mitochondria. To test the hypothesis that plant triterpenes may also induce mechanisms leading to reduced production of reactive oxygen species after induction with ethanol and acetaminophen, the cytoprotective effect of two plant triterpenes, betulin and its oxidized form, betulinic acid, was compared. Acetaminophen alone at concentrations between 25-250 µg/ml exhibited dose-dependent cytotoxicity for HepG2 cells. When a mixture of 25 µg/ml of acetaminophen was mixed with different concentrations of 5-50 mM of ethanol, an additional increase in the toxicity was observed, depending on the ethanol concentration used. Acetaminophen alone was detected as a strong inducer of superoxide anion in HepG2 cells. Moreover, the mixture of acetaminophen with ethanol induced significantly more superoxide anion than acetaminophen and ethanol alone. When betulin or betulinic acid were preincubated with HepG2 cells the level of superoxide anion and hydrogen peroxide was significantly lower than in respective controls. Betulin, in comparison to betulinic acid, was a stronger inhibitor of oxidative burst.