Gamma irradiation of DNA solutions containing copper causes changes in DNA conformation in oligonucleotides and in natural and synthetic DNAs. Diagnostic for these conformational changes is a ubiquitous 187-nm peak in the circular dichroism (CD) difference spectrum that has been predicted for a transformation from a right-handed to a left-handed helical DNA conformation. Changes in CD are correlated with changes in the UV spectrum. Reduction of DNA-bound Cu(II) to Cu(I) with ascorbic acid produces similar changes in CD spectra. These changes can be produced by the peroxy radical anion (O2*-) and the OH radical in the presence of copper. O2*- is approximately twice as efficient as *OH in initiating these changes in natural DNA. The changes in DNA conformation induced by ionizing radiation are remarkable in that they are dependent on the copper-ion concentration in a highly nonlinear manner at low copper concentrations and are not observed in the absence of copper ions. Possible implications of our results for radiobiological and oxidative damage in the cell nucleus are discussed.