Blood platelets in addition to their haemostatic role can function as inflammatory cells. The aim of our study was to assess if beta-D-glucan, the natural, very strong biological response modifier, may protect platelet proteins and lipids against oxidative/nitrative damages. The antioxidative activity of the beta-D-glucan, a known immunomodulator derived from the yeast cell walls of species such as Saccharomyces cerevisiae, on blood platelets treated with oxidants-peroxynitrite and hydroperoxide-was studied in vitro. The levels of different specific markers of oxidative stress, thiobarbituric acid reactive substances (TBARS), carbonyl groups and 3-nitrotyrosine (3-NT) were measured. Our studies showed that beta-glucan possesses significant defence properties against peroxynitrite or hydroperoxide induced lipid peroxidation. The level of TBARS was decreased by 80% at the highest dose of beta-glucan. In the presence of beta-glucan the distinct reduction of platelet protein oxidation was observed; the level of carbonyl groups was decreased by 50%. The results indicate that beta-glucan may also be effective in the protection against the nitrative action of peroxynitrite on platelet proteins, as in the presence of beta-glucan the level of 3-nitrotyrosine, measured by a competition-ELISA method, was diminished. The obtained in vitro results demonstrate that antiplatelet activity of beta-glucan from Saccharomyces cerevisiae is dependent on its antioxidative properties, and therefore beta-glucan supplementation may be beneficial in the prevention of excessive blood platelet activation-related diseases, such as cardiovascular or inflammatory diseases.