Most of the malignant tumors of man are not caused by only one specific carcinogen, but have to be attributed to additive effects of various carcinogenic factors, predominantly from the environment. Experimental studies in rats and mice showed syncarcinogenic effects of small doses of different carcinogens with the same organotropy, given either simultaneously or consecutively. These effects were proved in both local and systemic carcinogens. Extensive studies carried out in some thousand rats showed that the action of pre-or postnatally applied chemical carcinogens is not influenced by immunostimulating or immunodepressive measures. In view of the topicality of combination effects of chemical carcinogens and lacking influences by regulative mechanisms of the host organism, it is recommended to eliminate chemical carcinogens as far as possible from the human environment.