In order to study the leukemogenesis by a carcinogen chemical substance in newborn mice, various doses of benzpyrene (BP) were administered by a single subcutaneous injection into newborn RF mice, and hematological and histlogical analyses were made periodically afer the injection. Leukemia was induced in a high percentage of treated mice. They are all lymphocytic leukemia of thymic type, except one case of non-thymic type. A parallel correlation was observed between the doses of the carcinogen administered and the incidence of leukemia. The leukemic change occurred first in the cortical area of the thymus 1.5 months after BP injection, followed by the leukemic infiltration in various other organs and leukemic death in three months. Fibrosarcoma was also observed in the treated mice. Leukemia and fibrosarcoma were both transplantable with cell. Numerous C-type virus particles with budding were demonstrated by electron microscopy in the lymph nodes of BP-induced leukemic mice. From these findings, the author concludes that 1) the newborn mice are more susceptible than adult mice to the chemical carcinogen BP, 2) the thymus is the target organ and it plays an important role in the development of leukemia by BP, and 3) BP may induce leukemia by activating the provirus.