OBJECTIVES This work aims to evaluate the resistance of mononuclear cells to oxidative stress using a "KRL" test, formerly utilized to evaluate the resistance of erythrocyte to free radicals. METHODS The "KRL" test evaluates the resistance to lysis of cells treated by free radicals generated under standardized conditions. RESULTS We defined new analytical parameters (level of radical production, time course, number of cells) to obtain an accurate assay determining the resistance to oxidative stress of mononuclear cells, in comparison to that of erythrocytes. This test allows the evaluation of change in the redox state of mononuclear cells (improved by an antioxidant mix or deteriorated by antimycin A-induced mitochondrial radical overproduction). Interestingly, our data show that the sensitivity of mononuclear cells to oxidative stress is not correlated with the susceptibility of erythrocytes to oxidative stress. CONCLUSIONS The quantification of the susceptibility of mononuclear cells to oxidative stress gives additional information (in addition to erythrocyte resistance) and could be helpful for patients with chronic inflammation.