Acetamiprid, a neonicotinoid insecticide, is commonly used both in agriculture and domestic areas against a wide range of insects. The potential genotoxicity of a commercial formulation of acetamiprid (Mosetam 20 SP, containing 20% acetamiprid as the active ingredient) on human peripheral blood lymphocytes was examined in vitro by sister chromatid exchange (SCE), chromosomal aberrations (CAs), and micronucleus tests. Cells were treated with 25, 30, 35, and 40 mu g/ml of acetamiprid for 24 and 48 hr. Acetamiprid induced SCEs and CAs significantly at all concentrations and treatment times and micronucleus formation was significantly induced at 30, 35, and 40 mu g/ml of acetamiprid as compared with both the control and solvent control. Acetamiprid decreased the proliferation index (PI) at the two highest concentrations (35 and 40 mu g/ml) for the 24-hr treatment period and only at the highest concentration (40 mu g/ml) for the 48hr treatment period when compared with the control and solvent control. Peripheral lymphocytes exposed to all concentrations of acetamiprid showed significant decreases in mitotic index (MI) and nuclear division index (NDI) for both treatment periods when compared with both the control and solvent control. Furthermore, acetamiprid decreased the MI in both treatment periods, and the NDI only in the 24-hr treatment period to the same extent as the positive control, mitomycin C (MMC). This study presents the first in vitro evidence for the genotoxicity of a commercial formulation of acetamiprid in human peripheral lymphocytes.