A convenient "tube" assay to quantify relative antioxidant activities in aqueous solutions has been developed. Peroxynitrite was employed as a biologically relevant source of radicals with Pyrogallol Red as a detecting molecule. A variety of compounds have been examined, namely polyphenols, uric acid, glutathione, and ascorbic acid. Competition kinetics were observed for the majority of examined compounds, except thymol and ascorbic acid. Pyrogallol Red was fully protected by ascorbic acid against the bleaching by peroxynitrite until its total consumption. The deviation from competition kinetics in the case of thymol was due to the formation of radicals from thymol and their subsequent reaction with Pyrogallol Red. Quercetin was the most efficient scavenger of free radicals. The measurements of relative antioxidant activities using Pyrogallol Red and other detecting molecules, such as gallocyanine and carminic acid, were in fair agreement. The assay was successfully used for a screening of antioxidant activity of plant extracts of unknown composition.