Human granulocytes were capable of oxidizing 2-keto-4 thiomethylbutyric acid to ethylene during phagocytosis or membrane perturbation. The reaction required hydrogen peroxide and superoxide and in addition was inhibited by various hydroxyl radical (OH) scavengers. These observations represent direct evidence for the generation of OH by human granulocytes. Further, inhibition of ethylene generation by azide and cyanide suggests that OH generation in granulocytes may be linked to myeloperoxidase.