Cytotoxic and cell-transforming activities of methyl thiophanate a systemic fungicide capable of entering plant cells and thus controlling fungal diseases that have already started were studied in an in vitro medium-term (6-8 weeks) experimental model utilizing BALB/c 3T3 cells. Cells were exposed to the chemical, dissolved in dimethyl sulfoxide, in the absence or presence of an exogenous metabolizing system derived from rat livers supplemented with cofactors (S9 mix). In the absence of metabolic activation, methyl thiophanate exerted cytotoxic activity, evidenced through the formation of cell colonies, at low doses (> 10 micrograms/ml). However, the cytotoxic activity was greatly reduced by the S9 mix-induced metabolic activation of the chemical. Without bioactivation, cell-transforming potential, evidenced through the induction of transformation foci, was observable only at the highest (weakly toxic) dose employed (25 micrograms/ml). On the contrary, in the presence of metabolic activation, the cell-transforming activity was detectable at all tested doses (i.e. from 20 to 200 micrograms/ml) and it was particularly evident in a level-II transformation amplification test when the cells were allowed to perform active proliferative activity. These results, providing further information on the activity of methyl thiophanate in multistep carcinogenesis as possible genotoxic and/or co-carcinogenic agent, may contribute to better evaluate the oncogenic risk to man.