An epoxy derivative of linoleate, 9, 10-epoxy-12-octadecenoate, was demonstrated to be biosynthesized by leukocytes, thus nominated as leukotoxin. Its chemical structure was determined by gas-chromatography/mass spectrometry and nuclear magnetic resonance measurements. When it was injected intravenously, 15 mg/kg, canine heart showed signs of a typical cardiac failure; viz. Aortic flow started to drop immediately after the injection, and fell to 22% of the original at 40 min after the injection. At that point, systolic aortic pressure dropped to 35%, diastolic aortic pressure to 23%, and electronically differentiated maximal rate of left ventricular pressure rise (LV dp/dt) to 29%. All of experimental dogs died 40 to 50 min after the injection. On the contrary, administration of linoleic acid (15 mg/kg) did not affect these hemodynamical parameters. Therefore, leukotoxin seems to be an important factor to the genesis of heart failure.