THE use of tetrazolium compounds for the detection of certain reducing steroids on paper chromatograms1 prompted an inquiry into the possible application of these compounds to quantitative work, and a method of estimating corticosteroids has now been developed using 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride. Earlier attempts2 to utilize the intense blue colour formed on reduction of this substance in alkaline solution were not entirely successful, since it was not found possible to stabilize the colour sufficiently for routine work. The method has therefore been modified so as to make use of the more stable but slightly less intense red colour produced on neutralization of the blue reaction mixture. In addition, pyridine has been replaced by isopropanol as a more convenient solvent.