The question of determining doses of carcinogenic substances which are safe for man has great theoretical and practical value. The environment contains not a few carcinogenic active chemical mixtures of the most varied origins, and it is natural that hygienists, toxicologists and oncologists are interested in the possibility of establishing maximum permissible concentrations of such substances in the atmosphere, soil, drinking water, food products, etc. If carcinogenic substances, in very small doses do not actually cause the development of a tumor, do they create in the organism subjected to their influence heightened sensitivity to the effect of carcinogenic factors. Can answers to these questions regarding any group of carcinogens be attributed to all kinds of chemical carcinogenic substances, i.e. is the mechanism of the effect of carcinogens and the course of development of a precancerous condition and a malignant tumor under their influence of importance. The report attempts to answer some of these questions.