Zinc plays an essential role in a wide range of cellular processes, including defense against free radicals and maintaining genomic stability. The presence of zinc in some proteins is fundamental for their functioning as transcription factors. Little is known about interaction between zinc and DNA, which can be important in light of reports on the role of zinc in cancer transformation and sometimes contradictory character of these reports. In the present study we studied cyto- and genotoxicity of zinc sulfate (ZnSO4) in normal human lymphocytes and human myelogenous lukemia K562 cancer cells in the presence of zinc and hydrogen peroxide (H2O2). Zinc at concentrations from the range 10–1000 μM decreased the viability of cancer cells and this effect, especially for low concentrations of the element, was much more pronounced than in normal cells. Zinc did not induce DNA damage in normal cells, but did so in cancer cells. We observed a key difference between the action of zinc in normal and cancer cells in the presence of H2O2, since the element exerted a protective effect against cyto- and geno-toxic action of H2O2 in the former, whereas it increased such action in the latter. Zinc inhibited the repair of DNA damage induced by H2O2 in cancer cells. The results suggest that zinc may protect normal cells against DNA-damaging action and increase this action in cancer cells, which indicates the dual action of this element in dependency of target cells and can be useful in cancer therapy.