Phenanthroline and bipyridine, strong chelators of iron, protect DNA from single-strand break formation by H2O2 in human fibroblasts. This fact strongly supports the concept that these DNA single-strand breaks are produced by hydroxyl radicals generated by a Fenton-like reaction between intracellular Fe2+ and H2O2: H2O2 + Fe2+----Fe3+ + OH- + OH: Corroborating this idea is the fact that thiourea, an effective OH radical scavenger, prevents the formation of DNA single-strand breaks by H2O2 in nuclei from human fibroblasts. The copper chelator diethyldithiocarbamate, a strong inhibitor of superoxide dismutase, greatly enhances the in vivo production of DNA single-strand breaks by H2O in fibroblasts. This supports the idea that Fe3+ is reduced to Fe2+ by superoxide ion: O divided by 2 + Fe3+----O2 + Fe2+; and therefore that the sum of this reaction and the Fenton reaction, namely the so-called Haber-Weiss reaction, H2O2 + O divided by 2----O2 + OH- + OH; represents the mode whereby OH radical is produced from H2O2 in the cell. EDTA completely protects DNA from single-strand break formation in nuclei. The chelator therefore removes iron from the chromatin, and although the Fe-EDTA complex formed is capable of reacting with H2O2, the OH radical generated under these conditions is not close enough to hit DNA. Therefore iron complexed to chromatin functions as catalyst for the Haber-Weiss reaction in vivo, similarly to the role played by Fe-chelates in vitro.