The carcinogenic action of 3,4-benzpyrene (3,4-BP) can be reduced or completely inhibited by the presence of substances which occur in the animal organism and also by compounds prepared synthetically. Experiments in female mice have shown that the addition of some unsaturated aliphatic dicarboxylic and tricarboxylic acids, thiols or biogenic amines to 3,4-benzyprene solutions causes a slowing down of the tumor induction or complete blockade of the tumor development. The same 3,4-BP solutions without the addition of the substances mentioned produced malignant tumors in the mice almost without exception.