The evident growth of tumour morbidity makes analysis of the fundamental problems of neoplasm prophylaxis in man particularly urgent. The establishment of maximum permissible concentrations for carcinogenic agents calls for further exploration into the problem of threshold action of carcinogens and elaboration of a methodology for quantitative assessment of the biological effects of certain environmental factors. In particular, the available data on the ability of low-intensity environmental factors to modify the organism's general resistance and thereby produce in it conditions for materialization of the untoward effects of carcinogens invite further work in this direction. The study of environmental factors as possible modifiers of blastomatosis and investigation of the modification mechanisms involved are an immediate and topical task. Quantitative analysis of these modifying effects would allow establishment of well-grounded hygienic norms as the next step. Thus, study of the regularities involved, development of a fundamental methodology and accumulation of facts on the influence of low-intensity environmental factors on carcinogenesis and the cancer effect should be helpful in devising scientifically based measures of lowering the growth of tumour morbidity.