Halomethanes (HMs) can be formed during the chlorination process to obtain drinking water. In liver cells, HMs had been shown to be mutagenic and carcinogenic; however, their bioactivation by CYP 2E1 and GSTT1 is required. Although inhalation is the most common pathway of exposure, reports on the toxic effects induced by HMs in human lung are contradictory. The aim of this study was therefore to evaluate in vitro cytotoxicity and cell proliferation induced by CH(2)Cl(2), CHCl(3) and BrCHCl(2) in human lung NL20-TA epithelial cells and MRC-5 fibroblasts, and their relationship with CYP 2E1 and GSTT1 activity. High concentrations of these HMs induced cytotoxicity, particularly in cells treated with BrCHCl(2). Low concentrations of BrCHCl(2) stimulated hyperproliferation of fibroblasts, the most probable consequence of which is regenerative proliferation related to collagen induction. Fibroblasts exposed to BrCHCl(2) exhibited low levels of CYP 2E1 activity suggesting that released bromine is able to alter this activity by affecting the active site or auto regulating the activity itself. GSTT1 was up to ten times more active than CYP 2E1 in both cell lines, indicating that potential lung damage is due to formation of pro-carcinogens such as formaldehyde.