Pentachlorophenol (PCP) is a widely used biocidal compound with several industrial, agricultural and domestic applications. Although it has been shown to induce systemic toxicity and carcinogenesis in several experimental studies, the literature is scarce regarding its toxic mechanisms of action. Recent investigations in our laboratory have shown that PCP induces cytotoxicity and transcriptionally activates stress genes in human liver carcinoma (HepG2) cells [1]. We hypothesized that PCP-induced expression of stress proteins may play a role in the molecular events leading to toxicity and tumorigenesis in liver cells. To test this hypothesis, we performed the MTT-assay for cell viability, and the Western Blot and densitometric analyses to assess the expression of cellular protein including CYP1A1, c-fos, HSP70, and p53. Data obtained from the MTT-assay indicated a strong dose-relationship with respect to PCP cytotoxicity. The LD50 was computed to be 23.0 +/- 5.6 micrograms/mL. Western Blot and densitometric analyses also demonstrated a linear dose-response relationship with regard to CYP1A1 expression within the dose range of 0-50 micrograms/mL. However, a biphasic response was obtained with regard to HSP70, c-fos, and p53 expression, showing a peak induction at 25 micrograms/mL, and a drastic reduction in protein expression at 50 micrograms/mL, probably due to cell death at higher level of PCP exposure. At lower level of exposure, PCP was found to be mitogenic.