The carcinogenesis literature concerning the susceptibilities of the various animal species and humans to chemical carcinogens is reviewed. The fundamental issue is whether the different laboratory animal species used in the past and at the present are the appropriate systems to detect the carcinogenic properties of chemicals. It was concluded that the main problem is the presence of high to moderate incidences of spontaneously-appearing cancers in laboratory animals. These tumor incidences are, to a significant extent, either caused by genetic factors and/or viruses which render the systems excessively-susceptible to the carcinogenic action of a large proportion of chemicals. The same types of genetic factors and/or viruses do not exist in the human species. Consequently, the data obtained in the various animal species are exaggerated to an unrealistic proportion due to the inherent flawed designs, therefore, it has little bearing in the human species. It is further concluded that a new animal breeding program should be initiated with the specific aim of developing several tumor-free or near tumor-free species and strains of laboratory animals. Until then, a healthy state of mind is required to separate the reality from the fiction.