Biochemical and cytophotometric investigations have shown that the DNA content of tumors increased in comparison with the homologic normal tissues and that authors who have conducted investigations using cytophotometric methods have detected a direct correlation between the amount of DNA in the nuclei of tumor cells and the degree of the differentiation. Thus, with the increase in the duration of effect of a blastomogenic agent on animals, the relative amount of polyploid cells increases in the liver. At the same time, it was established that the DNA content of hepatic cells increases in the process of carcinogenesis in this organ. The indicated studies were devoted to an investigation of the DNA content in the nuclei of tumor cells or cells in the stage of malignization, and, therefore, the final stage of the investigation was the formed tumor.