The toxicity and carcinogenicity of drugs and other chemicals is, in most cases, mediated by highly reactive intermediates which are generated following metabolism catalysed by the enzymic apparatus of the exposed organisms. These reactive intermediates readily interact covalently with vital cellular components to provoke toxicity and carcinogenicity. The ubiquitous cytochrome P450-dependent mixed-function oxidases are the most important enzyme system in the activation of chemicals. This enzyme system comprises a number of families, each of which contains one or more subfamilies. The CYPIA and CYP2E subfamilies are the most closely associated with the production of reactive intermediates and, consequently, the manifestation of toxicity and carcinogenicity. A computer based molecular structure procedure (COMPACT) has been developed which, via a calculation of the molecular and electronic structure of the chemical, determines whether the chemical will interact with either of these two cytochrome P450 subfamilies and hence be metabolised to form reactive intermediates that manifest toxicity. As the basal levels of these two subfamilies are generally low, the ability of a chemical to induce them selectively, on repeated administration, is an important determinant of its toxic and carcinogenic potential. This inductive capability may be determined in short-term studies (ENACT) using only a small number of animals. Thus the combination of COMPACT and ENACT provides a rapid and inexpensive means for the preliminary screening of chemicals for toxicity and carcinogenicity before undertaking the long-term and expensive rodent lifetime bioassays.