AbstractIn proportion to the environmental pollution problems caused by organotin compounds, the genotoxicities of tin compounds in the environments have become of interest so as to estimate their safety in recent years. In this work, isolated λ‐DNA (double‐strand DNA) was incubated with inorganic tin(II) and tin(IV) and five organotin compounds [n‐butyltin trichloride, di(n‐butyltin) dichloride, methyltin trichloride, dimethyltin dichloride and trimethyltin chloride] in reaction systems both with and without hydrogen peroxide (H2O2) content. The tin compounds tested in this study did not induce DNA breakage in the absence of hydrogen peroxide. Divalent inorganic tin (SnCl2) and tetravalent inorganic tin (SnCl4) caused DNA breakage in the presence of hydrogen peroxide (10 mM), and the DNA damage activity of inorganic tin was much more potent in divalent inorganic tin (SnCl2) than in tetravalent inorganic tin (SnCl4). Divalent inorganic tin (SnCl2) induced DNA breakage in a concentration‐dependent fashion at concentrations greater than 0.1 mM of SnCl2in the presence of hydrogen peroxide (10 mM). DNA breakage was not caused by n‐butyltin compounds and methyltin compounds either in the presence or in the absence of hydro