The possible genotoxic effect of menthol was investigated by analysing the frequencies of sister chromatid exchange (SCE) and chromosomal aberrations in cultured human lymphocytes exposed to menthol. Phytohaemagglutinin-stimulated human lymphocyte cultures grown in the presence of menthol at final concentrations of 0.1, 1 or 10 mm, with or without S-9, had frequencies of polyploid cells and structural chromosomal aberrations similar to those seen in the solvent controls. Furthermore, menthol, either in the presence or absence of S-9, did not alter the SCE frequency in human chromosomes. These results suggest that menthol does not have a chromosomal-damaging effect in human lymphocytes.