Tachpyr (N,N'N"-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane), a novel metal chelator, was previously shown to deplete intracellular iron and exert a cytotoxic effect on cultured bladder cancer cells. Tachpyr binds Fe(II) and readily reduces Fe(III). The iron(II)-Tachpyr chelate undergoes intramolecular oxidative dehydrogenation resulting in mono- and diimino Fe(II) complexes. The present study investigates the redox-activity of the Tachpyr-iron complex to better define the mechanism of Tachpyr's cytotoxicity. Tachpyr's mechanism of cytotoxicity was studied using cell-free solutions, isolated DNA, and cultured mammalian cells by employing UV-VIS spectrophotometry, oximetry, spin-trapping technique, and electron paramagnetic resonance (EPR) spectrometry. The results show that: (1) Tachpyr by itself after 24 h of incubation had a cytotoxic effect on cultured cells; (2) fully oxidized Tachpyr had no cytotoxic effects on cultured cells even after 24 h of incubation; (3) Tachpyr protected isolated DNA against H(2)O(2)-induced damage, but not against HX/XO-induced damage; and (4) Tachpyr-Fe(II) chelate slows down but does not block oxidation of Fe(II), allows O*(-)(2)-induced or Tachpyr-induced reduction of Fe(III), and consequently promotes production of *OH through the Haber-Weiss reaction cycle. The results indicate that Tachpyr can protect cells against short-term, metal-mediated damage. However, upon prolonged incubation, Tachpyr exerts cytotoxic effects. Therefore, in addition to iron depletion, low-level oxidative stress, which in part occurs because of redox cycling of the coordinated iron ion, may contribute to the cytotoxic effects of Tachpyr.