Cyclooxygenase (COX, also called Prostaglandin H Synthase or PGHS) is a bifunctional enzyme that converts arachidonic acid to a hydroperoxy endoperoxide (PGG2) and reduces the endoperoxide to the corresponding alcohol (PGH2), the precursor of prostaglandins, thromboxanes, and prostacyclins. COX-1 is constitutively expressed in a variety of cell types and is involved in normal cellular homeostasis. The COX-1 (human) Inhibitor Screening Assay directly measures PGF2α by SnCl2 reduction of COX-derived PGH2 produced in the COX reaction. The prostanoid product is quantified via enzyme immunoassay using a broadly specific antiserum that binds to all the major prostaglandin compounds. This assay includes human recombinant COX-1 allowing the user to screen specific inhibitors. This assay is an excellent tool that can be used for general inhibitor screening, or to eliminate false positive leads generated by less specific methods.