While the liver consists of both parenchymal cells (PC) and nonparenchymal cells (NPC), virtually all studies on promutagen activation have been performed using PC. To evaluate the comparative roles of PC and NPC in promutagen activation, we cocultivated a cell line generally considered to have an insignificant level of xenobiotic metabolism, Chinese hamster ovary (CHO) cells, with either PC, NPC, or a combination of both. The mixed culture was treated with two promutagens: dimethylnitrosamine (DMN) and 3-methylcholanthrene (3-MC). The induction of 6-thioguanine resistant mutants was evaluated using the well-established CHO/hypoxanthine-guanine phosphoribosyl transferase (HGPRT) assay. Activation of promutagens, as indicated by an increase in mutant frequency in CHO cells, was observed only when the PC were present with the CHO cells during the treatment period. No activation was observed with NPC. Coculturing of PC and NPC yielded essentially the same results as PC alone. P-450 mixed function monooxygenase activity measured by the 7-ethoxycoumarin-O-deethylase assay further substantiates that PC had a significantly higher xenobiotic metabolism activity than NPC. Our study therefore indicates that PC, not NPC, are the major cell population in the liver responsible for the activation of promutagens.