Furazolidone is an antimicrobial compound used in human and veterinary medicine. The aim of this investigation was to determine its genotoxic capacity in vitro and in vivo. We used the human lymphocyte culture system to detect the effect of 2.0, 4.0, 6.0, 8.0, or 10.0 pg/ml, and the mouse bone marrow assay to determine the effect of 8.6, 30.0, or 75.0 mg/kg furazolidone. In both systems we determined the frequency of sister-chromatid exchanges (SCE), the cell proliferation kinetics (CPK), and the mitotic index (Ml). The in vitro results showed a significant SCE increase starting from the second dose tested and a CPK and Ml decrease starting from the third dose. The in vivo results showed a SCE increase with the two high doses tested, but no significant modification was found in the CPK and Ml with the three doses tested in the experiment.