The ferrous oxidation-xylenol orange (FOX) assay method for determination of lipid hydroperoxides is based on that under acidic conditions Fe2+ is oxidized to Fe3+, which then oxidizes xylenol orange to a product that absorb at 550 nm. The procedure has been adapted for determination of lipoxygenase activity in plant extracts. This enzyme is responsible for generation of off-flavors in vegetal foods, bleaching of pigments, and a lot of oxidative degradations. It is of interest to check the initial lipoxygenase activity in vegetal foods before the processing, using an assay that is rapid, reproducible, and easily adaptable to high throughput format. The enzymatic assay is based on a discontinuous determination of lipoxygenase activity using the FOX reagent for colorimetric determination of hydroperoxides accumulated in the medium by a period of incubation that is established by the addition of the extract (start of the reaction) and the addition of FOX reagent (finish of the reaction). The procedure is capable of detecting lipoxygenase activity in a number of vegetable homogenates, being especially useful for a rapid visual evaluation of this enzymatic activity.