Most chemical carcinogens require metabolic activation to electrophilic metabolites that are capable of binding to DNA and causing gene mutation. Carcinogen metabolism is carried out by large groups of xenobiotic-metabolizing enzymes that include the phase I cytochromes P450 (P450) and phase II enzymes that include various transferases. During the past 10 years, considerable attention has been focused on the role of P450s in human cancer susceptibility. Polymorphisms in expression of P450s and transferases exist in humans and these might render increased susceptibility or resistance to cancer. Thus it is important to understanding how P450s participate in the carcinogenesis process and to determine if they are indeed the rate limiting and critical interface between the chemical and its biological activity. Since there are marked species differences in expressions and catalytic activities of the multiple P450 forms that activate carcinogens, this validation process becomes especially difficult. To address the role of P450s in whole animal carcinogenesis, mice were produced that lack the P450s known to catalyze carcinogen activation. Mouse lines having disruption of genes encoding P450s CYP1A2, CYP2E1, and CYP1B1 were developed by use of gene disruption in empbryonic stem cells. Mice lacking expression of microsomal epoxide hydrolase and NADPH:quinone oxidoreductase were also made. These mice exhibit no grossly abnormal phenotypes, suggesting that the xenobiotic-metabolizing enzymes have no critical roles in mammalian development and physiological homeostasis. This explains the occurrence of polymorphisms in humans and other mammalian species. However, these null mice do show differences in sensitivities to acute chemical toxicities, thus establishing the importance of xenobiotic metabolism in activation pathways that lead to cell death. Rodent bioassays using null mice and known genotoxic carcinogens should establish whether these enzymes are required for carcinogenesis in an intact animal model. These studies will also provide a framework for the production of transgenic mice and carcinogen bioassay protocols that may be more predictive for identifying human carcinogens and validate the molecular epidemiology studies ongoing in humans that seek to establish a role for polymorphisms in cancer risk.