The COX Colorimetric Inhibitor Screening Assay measures the peroxidase component of cyclooxygenases. The peroxidase activity is assayed colorimetrically by monitoring the appearance of oxidized N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) at 590 nm. Inhibition of COX activity, measured by TMPD oxidation, by a variety of selective and nonselective inhibitors, shows changed potencies similar to those observed with other in vitro methods. The COX Colorimetric Inhibitor Screening assay includes both ovine COX-1 and human recombinant COX-2 enzymes in order to screen isozyme-specific inhibitors. The Cayman COX Colorimetric Assay is a time saving tool for screening vast numbers of inhibitors.