Tetrachloroethane (TTCE), pentachloroethane (PCE), and hexachloroethane (HCE) were tested in diploid strain (D7) of the yeast Saccharomyces cerevisiae in suspension test with and without mammalian metabolic activation (S9). TTCE, PCE, and HCE gave positive results on cells harvested from logarithmic growth phase; only PCE induced a significant increase (P less than or equal to .01) of mitotic gene conversion and point reverse mutation on cells from stationary growth phase with metabolic activation (S9). The in vivo effects on cytochrome P450 content (cyt. P450), pentoxyresorufin O-dealkylase (P450-like, class IIB, PROD), and ethoxy-resorufin O-deethylase (P448-like, class IA, EROD) activities were examined in hepatic microsomes from mice 24 h after acute intoxication. All the halogenated hydrocarbons displayed a marked toxic effect as shown by the significant decrease in cyt. P450 levels (maximum of 76% decrease, with TTCE 753.2 mg/kg) and EROD (maximum of 69% decrease, with PCE 925.4 mg/kg), and to a lesser extent in PROD (maximum of 52.4% decrease, with HCE 3150 mg/kg). Although a general decrease of P450 functions was observed, the toxic effects of TTCE and PCE seem to be preferentially related to P448 forms.