The sonicate of human neutrophils converted leukotriene B4 to a polar product in aerobic condition in the presence of NADPH at a rate comparable to that of the intact cells. NADH could scarcely replace NADPH. The conversion was not observed in anaerobic conditions and was inhibited by carbon monoxide (CO/O2 = 4/1) or by 1 mM p-chlormercuribenzoate, while it was not affected by 1 mM KCN, 5 mM NaN3, 200 micrograms/ml catalase, 100 mM mannitol, and 10 micrograms/ml superoxide dismutase. These observations suggest that the myeloperoxidase-H2O2-halide system and active oxygen species are not involved in the reaction. The activity was observed in the 100,000xg supernatant from the homogenate, in which cytochrome P-450 was not detected.