Amoxicillin (AMO), a drug used in the treatment of infections caused by susceptible bacteria, has been evaluated for its ability to induce genotoxicity in human peripheral blood lymphocytes. The potential genotoxic effects of AMO were investigated in vitro by the sister chromatid exchange (SCE), chromosomal aberration (CA), and micronucleus (MN) tests. The cells were treated with 400, 600, 800, and 1,000 mu g/ml AMO in the presence and absence of a metabolic activator (S9 mix), respectively. In this study, AMO did not induce SCEs or CAs in human peripheral blood lymphocytes both in the presence and absence of the metabolic activator. AMO concentration-dependently decreased the proliferation index (PI) in the absence of the metabolic activation for 24-hr treatment period. Mitotic index (MI) was generally found to have been reduced when compared with the negative control but not with the solvent control in cultures treated with AMO for 24 hr. AMO did not decrease the PI and MI in the presence of the metabolic activator. Furthermore, AMO neither induced the formation of MN nor decreased the nuclear division index in human peripheral blood lymphocytes both in the presence and absence of the metabolic activator. According to the present results, we suggest that AMO does not pose genotoxic risk for patients who are under therapy against bacterial infections. Environ. Mol. Mutagen. 51:222-228, 2010. (C) 2009 Wiley-Liss, Inc.