Neutrophils (PMNs) initiate production of toxic oxygen metabolites through stimulation of an NADPH oxidase resulting in the reduction of oxygen to superoxide anion (O2). The activity of this enzyme system may be primed by a variety of compounds. This laboratory investigated the possibility that stored blood components may contain agents which prime the PMN oxidase. At the time of outdate, packed red blood cells, whole blood, and platelet concentrates contained a priming agent which enhanced the PMN NADPH oxidase activity in response to a soluble stimulus, formyl-Met-Leu-Phe, by 2.1-to 2.8-fold. The priming activity was almost completely inhibited by WEB 2170, a specific platelet activating factor antagonist. Fresh plasma or fresh-frozen plasma did not exhibit priming activity. These data suggest that platelet-activating factor-like compounds are generated during the storage of cellular blood components. The presence of these agents in stored blood may suggest a role for specific componds which prime PMNs and possibly mediate other effects which result in severe complications of transfusion therapy.