The minor tobacco alkaloid myosmine is implicated in DNA damage through pyridyloxobutylation similar to the tobacco-specific nitrosamines (TSNA). In contrast to TSNA, occurrence of myosmine is not restricted to tobacco. Myosmine is genotoxic to human cells in the comet assay. In this study, the mutagenic effect of myosmine was evaluated using the cloning hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene mutation assay. Four hour exposure of isolated peripheral blood lymphocytes from 14 subjects homozygous for the Leu84 wild-type of the O6-methylguanine-DNA-methyltransferase (MGMT) gene to 1mM of myosmine increased mutant frequency from 0.73+/-0.58 x 10(-6) in control to 1.14+/-0.89 x 10(-6) lymphocytes (P<0.05). These new data further confirm the mutagenic effects of myosmine.