While the total antioxidant capacity (TAC) of blood plasma is mainly accounted for by urate, TAC of cell interior can be expected to depend more on other antioxidants, especially glutathione and protein -SH groups. We studied TAC of homogenates of several lines of cultured cells subjected to the action of thiol-modifying agents. Comparison of changes of TAC of the homogenates and of the level of total thiols (determined with a biradical spin label) demonstrates that alterations in cellular thiol content is the main determinant of changes of TAC of cell homogenates. These results show that estimation of TAC of cell extracts may be a useful parameter of assessment of oxidative stress, primarily of oxidation of thiol groups, yielding information different than TAC of body fluids.