A procedure is described for the separation of arachidonic acid and its metabolites: prostaglandins, leukotrienes, thromboxanes, and hetes. An online radioactive flow detector was used to screen supernatants of stimulated rat peritoneal macrophage for metabolites of arachidonic acid. Tritium labeled arachidonic acid metabolites were used as standards to identify (14)C-metabolites produced from stimulated rat peritoneal macrophage. Unlabeled standards were detected by UV absorption.