Plasmid DNA and restriction fragments of 80 and 120 base pairs were irradiated with fast neutrons in the presence of CuCl2. The number of single and double strand breaks is higher in the presence than in the absence of Cu2+ ions. The radiosensitizing effect was lower for solutions of high compared with low ionic strength, and also lower for deoxygenated than for aerated solutions. This effect was inhibited by EDTA, catalase and Tris, but not by ethanol. Superoxide dismutase partially inhibited the effect of low copper concentrations (< 1 Cu2+/nucleotide). Saturation of the solutions with N2O removed the effect for these concentrations of copper. The sensitization occurred preferentially at pyrimidines (thymines > cytosines) situated 5' to one or several purines (guanine > adenine) or located between two purines, at runs of purines (guanine > adenine), and at combinations of such sequences. The results can be only partially explained by a Fenton-like mechanism involving radiation induced hydrated electrons and hydrogen peroxide, which produces OH. radicals at the sites of binding of copper on DNA. The regions around these binding sites may undergo conformational changes. A second path for sensitization could be the enhancement of the efficiency of cleavage by the radiolytically produced OH. radicals in these conformationally modified regions.