The COX Activity Assay utilizes 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. The assay can be used to measure COX activity (COX-1 and COX-2) in cell lysates, tissue homogenates, and purified enzyme preparations. The assay includes COX-1 and COX-2 specific inhibitors in order to distinguish between the two isozymes.