Abstract: A test composition, device and method for determining a perioxidatively active substance in a test sample, as well as a method for preparing and using the device, are disclosed. The composition comprises an organic hydroperoxide, a benzidine indicator capable of providing a detectable response in the presence of the organic hydroperoxide and a peroxidatively active substance, and, as a stabilizing agent, an aniline chosen from pheny-1-naphthylamine, N,N-dimethylaniline and mixtures thereof. The device comprises a carrier matrix incorporated with the composition, and the method for using the device comprises contacting a test sample with the device and observing a detectable response therein. A method for preparing the device comprises preparing two or more solutions of the ingredients of the composition, wetting the carrier matrix sequentially therewith and drying the matrix after each wetting.