The hypoxic radiosensitizer 1-(2-hydroxy-3-methoxypropyl)-2-chloro-4-nitroimidazole [P40] was investigated for its mutagenic activity in bacterial Ames test as well as for genotoxic activity in micronucleus assay in vivo. This nitroimidazole showed the weak mutagenicity towards TA100 strain (base pair substitution) and towards TA98 strain (frameshift) only in the highest concentration. P40 induced also a significant increase in the frequency of micronucleated polychromatic erythrocytes (PCEs) at the doses of 0.6 mg/g and 1.2 mg/g. The maximum time response was at 48 h. The decrease of percentage of PCEs suggested the possible cytotoxicity on bone marrow cells after treatment with P40. Positive results in this battery short-term tests provide evidence of clastogenic activity of P40.