AbstractThe metabolism of chlorpyrifos (CPS) and chlorpyrifos oxon (CPO) by human hepatocytes and human liver S9 fractions was investigated using LC‐MS/MS. Cytochrome P450 (CYP)‐dependent and phase II‐related products were determined following incubation with CPS and CPO. CYP‐related products, 3,5,6‐trichloro‐2‐pyridinol (TCP), diethyl thiophosphate, and dealkylated CPS, were found following CPS treatment and dealkylated CPO following CPO treatment. Diethyl phosphate was not identified because of its high polarity and lack of retention with the chromatographic conditions employed. Phase II‐related conjugates, including O‐ and S‐glucuronides as well as 11 GSH‐derived metabolites, were identified in CPS‐treated human hepatocytes, although the O‐sulfate of TCP conjugate was found only when human liver S9 fractions were used as the enzyme source. O‐Glucuronide of TCP was also identified in CPO‐treated hepatocytes. CPS and CPO were identified using HPLC–UV after CPS metabolism by the human liver S9 fraction. However, CPO was not found following treatment of human hepatocytes with either CPS or CPO. These results suggest that human liver plays an important role in detoxification, rather than activation, of CPS. © 2006 Wiley Periodicals, Inc. J Biochem Mol Toxicol 20:279–291, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jbt.20145