A large number of chemical carcinogens require metabolic activation before they are biologically active. The metabolism of a few of these compounds has been investigated in cultured human tissues and cells and the metabolism has been compared with the metabolism in organs from experimental animals in which the compounds induce cancer. Generally, only quantitative differences could be observed between animal and human tissues. The development of new methods to detect carcinogen-DNA adducts makes it feasible to study and compare the metabolism in human and animal tissues of an increasing number of potential human carcinogens. Furthermore, construction of cell lines, expressing the human forms of the carcinogen metabolizing enzymes, by biotechnology provides a new model to study the metabolism and to monitor for genetic markers in the same cells.