The action of two phenolic compounds isolated from the bark of Yucca schidigera: trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene and its analogue -- resveratrol (trans-3,4',5-trihydroxystilbene, present also in grapes and wine) on oxidative/nitrative stress induced by peroxynitrite (ONOO(-), which is strong physiological oxidant and inflammatory mediator) in human blood platelets was compared. The trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene, like resveratrol, significantly inhibited protein carbonylation and nitration (measured by enzyme-linked immunosorbent assay method) in the blood platelets treated with peroxynitrite (0.1 mM) and markedly reduced an oxidation of thiol groups of proteins (estimated with 5,5'-dithio-bis(2-nitro-benzoic acid)] or glutathione (measured by high performance liquid chromatography method) in these cells. The trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene, like resveratrol, also caused a distinct reduction of platelet lipid peroxidation induced by peroxynitrite. The obtained results indicate that in vitro trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene and resveratrol have very similar protective effects against peroxynitrite-induced oxidative/nitrative damage to the human platelet proteins and lipids. Moreover, trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene proved to be even more potent than resveratrol in antioxidative tests. We conclude that the novel tested phenolic compound -- trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene isolated from Y. schidigera bark possessing Generally Recognized As Safe label given by the Food and Drug Administration and allows their human dietary use -- seems to be a promising candidate for future evaluations of its antioxidative activity and may be a good candidate for scavenging peroxynitrite.