Activated neutrophils and monocytes were found to metabolize procainamide to a reactive hydroxylamine. In contrast, there was little or no metabolism by lymphocytes or platelets. Therefore, it appears that only leukocytes that contain myeloperoxidase can metabolize procainamide to a significant degree. There was no difference in the degree to which neutrophils from males or females metabolized procainamide; however, monocytes from males formed significantly more hydroxylamine than did monocytes from females. By use of radiolabeled procainamide, covalent binding of procainamide to leukocytes was detected, and the degree of binding correlated with the cells' ability to oxidize procainamide. These findings suggest that myeloperoxidase is the major enzyme involved in the formation of reactive metabolites by leukocytes, a pathway that we propose may be responsible for procainamide-induced lupus and agranulocytosis.