[en] Local factors such as proliferative activity and the numerical ratio between epithelial and mesenchymal cells, and also the character of interaction between the tissue components in ontogeny may play an important role in the realization of sensitivity of mice of a particular line to the development of lung tumors. These characteristics of lung tissue in mice of lines A and C57BL are investigated under normal conditions and during induced carcinogenesis. Results are given of a comparative study of the relative numbers of epithelial and mesenchymal cells in organ cultures of embryonic lungs. ³H-thymidine was added to the cultures on the 14th day of the experiment in a concentration of 1 microCi/m1 medium. An autoradiographic study of the cultures was performed