It is known that histones and other DNA-binding polycations protect DNA from radiation damage mediated by hydroxyl radicals. Until recently, this protection of DNA has mainly been attributed to compaction and aggregation. It was hypothesized that chemical repair of DNA sugar radicals by donation of hydrogen atom from polycations also significantly contributes to DNA protection. To test this hypothesis, the relative yields of low-molecular weight characteristic products of oxidation of DNA sugar were compared in X-irradiated samples of naked DNA and DNA complexes with a number of polycations by using an HPLC-based method of DNA damage product quantification. The variation in the percent contribution of the C1„ sugar damage product ongoing from free DNA to DNA-polycations complexes is in agreement with the hypothesis that chemical repair of DNA sugar radicals by donation of hydrogen atom from polycations contributes to the overall DNA protection against hydroxyl radical-mediated damage.