We compared the antioxidant activity of serum and plasma samples of a known glutathione content with the activity of glutathione, whilst determining to what extent various stress factors might change the activity of the tested samples. Copper ions and benzene were used as examples of environmental stress factors, and xenobiotics in the form of representatives of various groups of drugs, were used as examples of pharmacological stressors at therapeutic ranges. The activity was assessed by the ABTS, ORAC, FRAP and CUPRAC methods. Glutathione content was measured by the HPLC-FD method. During the experiments, plasma samples were shown to be more resistant to oxidative stress. Moreover, the important role of environmental xenobiotics in oxidative stress was revealed, as well as the differentiated influence of pharmaceutical xenobiotics. Among all pharmaceutical xenobiotics tested, including representatives of antiarrhythmic, antiepileptic, cytostatic and mucolytic drugs, the greatest stress was shown for antiarrhythmic drugs and cytostatics. [Display omitted] • Antioxidant activity (AA) of serum, plasma and glutathione samples was compared. • Differentiated impact of various exogenous stress factors on AA was presented. • The greatest impact of antiarrhythmic and cytostatics drugs was proved. • The highest resistance to stress factors was demonstrated for human plasma. • It was proved that Acethylocysteine evidently promotes the increase of AA. [ABSTRACT FROM AUTHOR]