Two hepatoma cell cultures were examined as in vitro models to be used in genotoxicity and cytotoxicity tests without the addition of bioactivating enzymes. The MH1C1, and HTC hepatoma lines were used in this study to establish their sensitivity to a number of xenobiotics, namely, cyclophosphamide (CP), the classical positive control in bioactivation tests; benzaldehyde (BA), a short-chain aldehyde; and 4-hydroxynonenal (HNE), a major toxic end-product of the peroxidative degradation of cell membrane lipids. As a first approach, we compared the following cytotoxicity tests: release of lactate dehydrogenase (LDH), and colony formation efficiency (CF). Colony-forming cells were exposed to the drugs according to different procedures, before or after the anchorage phase. The leakage of LDH into the medium following exposure of both cell lines to HNE, CP and BA for up to 24 hours was found not to be a good index of cytotoxicity. A better indicator of cytotoxicity was CF, as evaluated by exposure of the cells 24 hours after seeding. The effects were detectable at very low concentrations, corresponding to 10, 90 and 100μM for HNE, CP and BA, respectively. The impairment of CF efficiency was dose-dependent and time-dependent, and several differences between the two cell lines were observed.