Glutathione Peroxidase (GPx) family of enzyme protects cell from oxidative stress. GPx converts reduced glutathione (GSH) to oxidized glutathione (GSSG) while reducing lipid hydroperoxides to their corresponding alcohols or free hydrogen peroxide to water. Low levels of GPx have been linked to free radical related disease. In the Glutathione Peroxidase Activity Colorimetric Assay, GPx reduces Cumene Hydroperoxide as well as oxidizing GSH to GSSG. The generated GSSG is reduced to GSH with consumption of NADPH by GR. The decrease NADPH can be measured at 340 nm and is proportional to GPx activity. The assay can be used to detected glutathione dependent peroxidaes in tissue homogenates, plasma and cell lysates. The detection limit is approx. 0.5 mU/mL of GPx in samples.