Reactive oxygen species and other free radicals are known to be the mediators of phenotypic and genotypic changes that lead from mutation to neoplasia. The imbalance in tumor cell antioxidant defense mechanism can influence also the sensitivity to cytoreductive therapy. In erythrocytes it can result to hemolysis which is one of the pathogenetic mechanisms of anemia in cancer patients. Parameters of lipid peroxidation (malondialdehyde-MDA) and antioxidant enzymes here represented by superoxide dismutase (SOD) and glutathione peroxidase (GPx) in multiple myeloma (MM) have been investigated. Nine patients of various clinical stages and activities of the disease were studied. Significantly higher concentrations of total MDA in plasma (1.20 +/- 0.24 mumol/l vs. 0.64 +/- 0.22 mumol/l, p < 0.0001) as well as in erythrocytes (2.72 +/- 0.81 mumol/l vs. 1.03 +/- 0.44 mumol/l, p < 0.0001) were found comparing to the control group. The levels of free MDA in plasma (0.31 +/- 0.09 mumol/l vs. 0.49 +/- 0.17 mumol/l, p < 0.05) and in erythrocytes (0.29 +/- 0.20 mumol/l vs. 0.59 +/- 0.22 mumol/l, p < 0.001) were decreased in myeloma patients. Significantly lower activities of GPx (19.17 +/- 4.07 U/g Hb vs. 23.26 +/- 3.61 U/g Hb, p < 0.05) and SOD (1882.46 +/- 181.73 U/g Hb vs. 2347.13 +/- 502.51 U/g Hb, p < 0.05) in erythrocytes were found. We did not observe evident relationship between the concentration of MDA or the activities of SOD and GPx and either the stage of the disease, or the level and the type of paraprotein. These results propose possible role of free radicals with reduced antioxidant activities of SOD and GPx in multiple myeloma.