Compounds with antioxidant activity may protect different elements of blood from damaging effects of oxidative stress. The present in vitro study was designed to examine the antioxidant activity of methanolic extracts from leaves and roots of R. glutinosa plants against damages induced by oxidative stress. Oxidative stress was stimulated in human plasma and human blood platelet samples by the strong biological oxidant--hydrogen peroxide (H2O2) or H2O2/Fe (the donor of hydroxyl radicals). In experimental trials, the level of biomarker of lipid peroxidation--TBARS was significantly decreased by the action of methanolic extracts from R. glutinosa leaves. Therefore, it seems that the methanolic extract from leaves of the plants may be a new source of bioactive antioxidant natural compounds. It may be also an active pharmacological agent or a food supplement for healthy subjects and for people with different diseases (cardiovascular diseases and cancer) induced by oxidative stress.