Methanogenic archaeon Methanobrevibacter arboriphilus (strains AZ and DH1), which is a strict anaerobic microorganism not able to synthesize heme, possessed a very high catalase activity in the presence of 20-50 mu M hemin in a growth medium. We investigated the effect of various oxidative stresses (hydrogen peroxide and oxygenation) on M. arboriphilus cells grown on the standard nutrient medium supplemented with 0.1 % yeast extract, and on the same medium supplemented with hemin. It was demonstrated that 30 mu M hemin had a very significant positive effect on the resistance of M. arboriphilus strains to H2O2 and O-2 stresses because of 30- to 40-fold increase of heme catalase activity. Thus, hydrogen peroxide (0.6-1.2 mM) or oxygen (3-5 %) had a strong negative impact on low-catalase cultures grown in the hemin-free standard medium, whereas the presence of 30 mu M hemin in the medium results in a high yield of biomass even under conditions of four times stronger H2O2 and two times stronger O-2 stresses. The intracellular catalase activity reached a high level in 30-60 min after hemin was added to the nutrient medium, but the activity already increased about 5-7-fold in 6 min after hemin addition. Our experimental data suggest that exogenous hemin provides an effective antioxidative defense in representatives of the genus Methanobrevibacter, specially playing an important role in the puromycin-insensitive formation of the active heme-containing catalase from presynthesized apoenzyme and heme.