Non-pedigree rats were injected subcutaneously either overheated oil or the oil and one of unsaturated fatty acids (linoleic, linolenic, oleic acids), or overheated oil and fresh oil. The greatest number of tumors in earlier terms occurred in the group of rats not receiving fatty acids. In simultaneous injection of overheated sunflower oil and fatty acids (oleic and linolenic), as well as fresh oil, tumors would arise considerably more rarely. Whereas, in injection of overheated sunflower oil and linoleic acid tumors did not arise at all. The mechanism of an inhibitory action of fatty acids on tumor development is discussed.