The public has developed the impression that in an industrialized society environmental carcinogens are ubiquitous and that all chemical exposures, irrespective of the dose, influence carcinogenesis. Such impressions are influenced by many nonscientific considerations and often reflect insufficient knowledge of scientific data and methods. Such misconceptions are thus partly explained by the multistage nature of carcinogenesis, the absence of a clear understanding, and definition of the multiple factors which influence cancer induction. It is obvious that not all chemicals are carcinogenic and it is reasonable to accept that not all substances (including primary chemical carcinogens) that are found to enhance carcinogens in animals are necessarily carcinogenic to humans under conditions of usual human exposure.