Six halogenated hydrocarbons, chloroform, 1,2-dibromoethane (1,2-DBE), 1,1-dichloroethane (1,1-DCE), 1,2-dichloroethane (1,2-DCE), 1,1,1-trichloroethane (1,1,1-TCE), and 1,1,2-trichloroethane (1,1,2-TCE), were evaluated for their cytotoxicity in primary cultures of rat hepatocytes isolated from normal, partially hepatectomized, and preneoplastic/neoplastic rat livers. Preneoplastic/neoplastic lesions of phenotypically altered foci and hepatocyte nodules were induced by either (1) initiation by diethylnitrosamine (DENA) followed by 2 weeks of 0.02% 2-acetylaminofluorene (2-AAF) in the diet and a single gavage dose of carbon tetrachloride 1 week after the start of the 2-AAF diet or (2) initiation by DENA followed by promotion with 500 ppm sodium phenobarbital in the drinking water for 24 weeks. The hepatocytes containing preneoplastic/neoplastic cells isolated from animals treated with either protocol, compared to hepatocytes isolated from normal liver, were resistant to the cytotoxicity of aflatoxin B1 (AFB1). None of the six halogenated alkanes exhibited any difference in their cytotoxicity toward hepatocytes isolated from normal liver or from liver containing preneoplastic/neoplastic lesions induced by either procedure. Hepatocytes isolated from partially hepatectomized animals were resistant to the cytotoxicity of AFB1 and chloroform but not to the cytotoxicity of 1,2-DBE or 1,2-DCE. The ranking of relative cytotoxicity in hepatocytes from untreated rats was 1,2-DBE much greater than 1,2-DCE greater than 1,1,2-TCE greater than 1,1,1-TCE greater than chloroform greater than 1,1-DCE. Treatment with SKF-525A protected the hepatocytes from the cytotoxicity of AFB1 while increasing the cytotoxicity of all six halogenated alkanes. Treatment with diethyl maleate increased the cytotoxicity of AFB1 and all six halogenated alkanes. These observations suggest that preneoplastic/neoplastic rat hepatocytes are not resistant to the cytotoxicity of the six halogenated alkanes because their toxicity might be mediated by a cytochrome P-450 species which is not inhibited by SKF-525A and is not decreased in preneoplastic/neoplastic lesions.