Electrolytic reduction of the hypoxic tumour cell radiosensitizing drug misonidazole was carried out at a controlled potential under anaerobic conditions in the presence of Escherichia coli DNA. During the reduction process the DNA was examined by viscometry, thermal hyperchromicity, melting and renaturation profiles, hydroxyapatite chromatography, agarose gel electrophoresis and alkaline sucrose density gradient centrifugation. The reduced drug decreases the viscosity, hyperchromicity and renaturation of DNA. These effects are consistent with strand breakage of the molecule which was corroborated by finding an increase in the single-strand content of DNA, increased migration and loss of fluorescence intensity on agarose gels and sedimentation to a less dense region in alkaline sucrose density gradients. The results are discussed in relation to postulated mechanisms of the selective toxicity of the drug towards anaerobes and cytotoxicity of electron affinic radiosensitizers of hypoxic tumour cells.