The present work demonstrates that cDNAs coding for cytochrome P450 enzymes can be transfected into mammalian cells and expressed. In the present studies, two different cell systems were used for transfection: 10T1/2 cells which can be used to study initiation and promotion (Diamond, 1984) and AHH-1 cells which can be used to study mutation and clastogenesis (Crespi and Thilly, 1984, Crespi and Penman, 1989). Thus, a diversity of endpoints can be studied in cells which have increased metabolic capability. By increasing the metabolic capability of the target cell, the effects of nongenotoxic as well as genotoxic chemicals, can be examined in the appropriate in vitro systems. For example, the 10T1/2 cells can be treated with a nontransforming dose of an initiator followed by continuous treatment with a second chemical that requires cytochrome P450 specific metabolism to manifest its promoting activity. By this approach, greater insight into the role of chemical metabolism in the promotion process (and presumably other nongenotoxic effects) can be obtained. Additionally, the role of specific cytochrome P450s in the metabolism of different classes of carcinogens/drugs can be elucidated. A major advantage of having the metabolizing enzymes actually present in the target cell is that effects of chemicals can be studied in long-term, low-dose exposure protocols which will eliminate the acute toxic effects which are associated with many current protocols. Thus, more realistic environmental exposure conditions can be achieved by using these in vitro systems containing endogenous metabolism systems.