Oleic acid is a monounsaturated fatty acid and is one of the major components of membrane phospholipids. Oleic acid contributes about 17% of the total fatty acids esterified to phosphatidylcholine, the major phospholipid class in porcine platelets. Oleic acid inhibits collagen-stimulated platelet aggregation by approximately 90% when used at a concentration of 10 µg/ml. fMLF-induced neutrophil aggregation and degranulation is inhibited by 55% and 68%, respectively, using 5 µM oleic acid. This inhibition is comparable to that observed using arachidonic acid under the same conditions. Oleic acid, whether applied extracellularly (EC50 = ~60 µM) to human platelets or released from membrane phospholipids, causes an increase in intracellular calcium levels.