Further information was gathered on the possible carcinogenic hazard associated to the exposure to the insecticide lindane (gamma-hexachlorocyclohexane). The parameters studied were the cytotoxic and cell transforming activities of the pesticide on BALB/c 3T3 cells in an in vitro experimental model system in the absence or in the presence of rat liver S-9 mix-induced metabolic activation of the chemical. Lindane did not exert cytotoxic effects at all the tested doses (ranging from 10 micrograms/ml to 200 micrograms/ml) in the absence of bioactivation. However, dose-related cytotoxic effects were observed in the presence of the metabolizing system. Furthermore, lindane showed statistically significant and dose-dependent cell transformation activity at all the tested doses (10 micrograms/ml, 50 micrograms/ml and 100 micrograms/ml ) either in the absence or in the presence of bioactivation. This activity was related with cell proliferation since it was exerted in a level-II transformation test by replating cells and allowing the amplification of the cell transforming effects of the chemical. The formation of radicals and of reactive oxygen species, resulting from the chemical metabolism, could account for lindane activity as carcinogenesis promoting agent, although contemporary genotoxic effects induced by the pesticide could not be excluded.