[Role of exogenous and endogenous factors in the etiology of human tumors].

The factors of major importance in the development of human tumors: exogenous chemical carcinogens, endogenous carcinogens, hormones, radiation, immunity disorders, heredity, nutrition, harmful habits are analysed on the basis of epidemiological and experimental evidence. Close interaction of exogenous and endogenous factors in the process of tumor development is demonstrated. Occupational carcinogens affecting at high doses small human contingents are frequently the major etiological cause of tumor development. In the general population exposed to the effect of quite low doses of carcinogens the latter are probably only initiators of neoplastic process completed by specific and nonspecific stimulators of carcinogenesis (cocarcinogens, promoters). The mechanisms of action of modifying factors on carcinogenesis (the effect of carcinogens on endogenous synthesis and metabolism, their interaction with cell macromolecules, tumor growth) are discussed.