Thein vitro antioxidant capacity of sulfasalazine (SASP), its metabolites (SP, 5-SSA), and olsalazine (OAZ), was studied by evaluating their effects on superoxide (O₂ ^– ) production. Assay systems were the xanthine-xanthine oxidase (X/XOD) reaction and phorbol myristate acetate (PMA)-activated polymorphonuclear leukocytes (PMNs), using the cytochromec (cyt-c) reduction assay and a luminol-dependent chemiluminescence method. 5-ASA, SASP, and OAZ showed a dose-dependent scavenger effect in both O₂ ^– generating systems, 5-ASA being the most powerful (>50% of inhibition in the PMNs system and >70% in the X/XOD system at 10 M concentration). SP had an inhibitory effect only in the PMNs system but did not modify the activity of xanthine oxidase, thus excluding a scavenger action. These data suggest that the scavenger effect of 5-ASA, SASP, and OAZ may be an important mechanism of action.