Cell lines derived from Reuber H-4-II-E hepatoma cells and their hybrids that differ in the expression of liver-specific functions are shown to contain different forms of monooxygenases. According to the specificity toward the substrates benzo(a)pyrene, aldrin and chenodeoxycholic acid, the kinetics of the epoxidation of aldrin, the response to inducers, such as benz(a)anthracene and dexamethasone, and the in vitro modifier 7,8-benzoflavone, the monooxygenases predominating in differentiated cell lines belong to the cytochrome P-450-dependent enzyme(s), those in the less differentiated lines belong to the cytochrome P-448-dependent form(s).