Cyclooxygenase (COX) converts arachidonic acid to prostaglandin (PG) H 2 , which is further metabolized to various prostaglandins, prostacyclin and thromboxane A 2 . COX exists in at least two different isoforms. COX-1 is constitutively expressed, whereas COX-2 is induced by proinflammatory stimuli. Prostaglandin E 2 is a major metabolite of COX activation. In order to compare the activity of target ligands and COX inhibitors on PGE 2 synthesis and release, the responsiveness of several cell lines to the calcium ionophore A23187, bacterial lipopolysaccharide (LPS), nonsteroidal anti-inflammatory drugs (NSAIDs), and the glucocorticoid, dexamethasone, were investigated. For intracellular measurements, the culture supernatant was aspirated, and the cells were thoroughly washed and lysed with dodecyltrimethylammonium bromide. Intracellular and secreted PGE 2 were measured with an enzyme immunoassay. A23187 and LPS increased intracellular PGE 2 in a dose-dependent manner. Kinetic experiments with A23187-stimulated mouse 3T3 fibroblast cells revealed a distinct biphasic response in COX activity. In the presence of NSAIDs or dexamethasone, there was a dose-dependent inhibition in intracellular PGE 2 with A23187-stimulated 3T3 cells. Inhibitory studies demonstrated an apparent increased sensitivity of COX activity to the action of inhibitors when measuring intracellular PGE 2 compared with using cell culture supernatants. Indeed, intracellular PGE 2 levels were comprehensively reduced in the presence of low concentrations of inhibitor. The utilization of cell culture lysates and, in particular, measurement of intracellular PGE 2 should prove useful for identifying new COX inhibitors.