Hemoglobin is considered a potentially toxic molecule when released from erythrocytes during hemolysis, inflammation, or tissue injury. The mechanisms of toxicity involve reduced nitric oxide bioavailability and oxidative processes both occurring at the heme prosthetic groups. When the endogenous oxidant H(2)O(2) reacts with Hb, transient radicals are generated during the peroxidative consumption of H(2)O(2). If not neutralized, these radicals can lead to tissue toxicity. The net biologic effect of extracellular Hb in an H(2)O(2)-rich environment will therefore be determined by the balance of H(2)O(2) decomposition (potential protective effect) and radical generation (potential damaging effect). Here we show that Hb can protect different cell types from H(2)O(2)-mediated cell death and the associated depletion of intracellular glutathione and ATP. Importantly, Hb blunts the transcriptional oxidative-stress response induced by H(2)O(2) in human vascular smooth muscle cells (VSMCs). Based on spectrophotometric and quantitative mass spectrometry analysis, we suggested a novel mechanism in which Hb redox-cycles H(2)O(2) and simultaneously internalizes the radical burden, with irreversible structural globin changes starting with specific amino acid oxidation involving the heme proximate betaCys93 and ultimately ending with protein precipitation. Our results suggest that complex interactions determine whether extracellular Hb, under certain circumstances, acts a protective or a damaging factor during peroxidative stress conditions.