Butachlor, a pre-emergence herbicide was investigated for its ability to induce sister chromatid exchanges (SCE) and chromosome aberrations (CA) in cultured human peripheral blood lymphocytes. Mitogen-stimulated lymphocytes were treated with three different concentrations (5, 10 and 20 μg/ml) of butachlor for 24, 48 and 72 h. Our results indicate a dose-dependent increase in the frequency of chromosomal aberrations at 24, 48 and 72 h of treatment with butachlor. No SCE was promoted by butachlor.