A clinical study was undertaken to evaluate the influence of a preparation containing selenium and vitamins with antioxidant properties (Protecton Zellaktiv from Smith Kline Naturarznei-Germany) on: (a) concentration of Se in serum and hair, (b) activity of glutathione peroxidase (GSH-Px), and (c) concentration of malondialdehyde (MDA) in serum, before and during supplementation. Women undergoing chemotherapy for ovarian cancer received selenium during three months at a daily dose of 200 micrograms. The results were compared with controls not receiving selenium supplementation. Administration of selenium significantly increased the serum and hair concentrations of selenium in the study group. After three months of supplementation the activity of GSH-Px was significantly higher in the study group. After one month of supplementation a difference in the concentration of MDA between the study and control groups was found. On the basis of the present results the administration of selenium in patients with ovarian cancer undergoing multi-drug chemotherapy is recommended.