Cyclooxygenase (COX, also called Prostaglandin H Synthase or PGHS) enzymes contain both cyclooxygenase and peroxidase activities. COX catalyzes the first step in the biosynthesis of prostaglandins (PGs), thromboxanes, and prostacyclins, the conversion of arachidonic acid to PGH2. It is now well established that there are two distinct isoforms of COX. Cyclooxygenase-1 (COX-1) is constitutively expressed in variety of cell types and is involved in normal cellular homeostasis. A variety of mitogenic stimuli such as phorbol esters, lipopolysaccharides, and cytokines lead to the induced expression of a second isoform of COX, cyclooxygenase-2 (COX-2). COX-2 is responsible for the biosynthesis of PGs under acute inflammatory conditions. This inducible COX-2 is believed to be the target enzyme for the anti-inflammatory activity of nonsteroidal anti-inflammatory drugs. The COX (ovine/human) Inhibitor Screening Assay directly measures PGF2alpha produced by SnCl2 reduction of COX-derived PGH2. The prostanoid product is quantified via enzyme immunosorbent assay (ELISA) using a broadly specific antibody that binds to all the major prostaglandin compounds. Thus, the Cayman COX assay is more accurate and reliable than an assay based on peroxidase inhibition. The Cayman COX Inhibitor Screening Assay includes both ovine COX-1 and human recombinant COX-2 enzymes in order to screen isozyme-specific inhibitors. This assay is an excellent tool which can be used for general inhibitor screening, or to eliminate false positive leads generated by less specific methods.