In sections of rat tumors arisen after transplacental and postnatal exposure to BP, DMVA, and NEM there was found the activity of NAD. H2-tetrazole-oxydoreductase, hexokinase, succinate and lactate dehydrogenase, glucoso-6-phosphate, alpha-glycerophosphate, glutamate, acid and alklaine phosphatase. A comparison of the histological structure and histochemical properties of malignant tumors showed their being independent on the chemical structure, dosage and terms of the carcinogen administration. However, the malignant signs of tumors of various histogenesis varied not only between epithelial, neurogenic and mesenchymal type of tumors but also within each of the types concerned.