Heme and porphyrins catalyze the formation of various reactive oxygen species under widely different conditions. Porphyrins are potent photosensitizers capable of transferring the energy of their excited state to oxygen, forming 1O2. Heme, by virtue of its iron content, is not a photosensitizer, but it can react with H2O2 forming reactive oxygen intermediates whose nature depends on the oxidation state of the iron. Ferric heme gives rise to a porphyrin cation radical, while ferrous heme catalyzes the formation of OH.. The reactive species formed by heme and porphyrins oxidize susceptible functional groups on protein and lipid components of membranes as well as serum and cytosolic proteins. Oxidation of membrane-bound proteins occurs independently of lipid peroxidation and it is characterized by extensive cross-linking, which takes place subsequent to amino acid oxidation. Serum and cytosolic proteins that bind and may transport heme and porphyrins are of special interest since they can modulate their toxicity by affecting their availability and reactivity. In addition, these proteins are particularly susceptible to oxidation, especially by heme, due to their proximity to the oxidizing species that are formed.