In order to study the carcinogenicity and its mechanism of N-Nitrosobutylurea (NBU), 10 week old RF mice was abministrated with 0.025% NBU water solution as drinking water every day. 1. Thymoma was induced after an average latency of 93 days and average rate of the tumor induction was 88.6%. Both sexes were equally susceptible. 2. Histologically thymoma was lymphosarcoma and other lymphatic organs also showed lymphosarcoma. However leukemic signs were not found in the peripheral blood and by clinical tissue culture. 3. Leukemia virus was not found in the electron microscpic observation and cell free transmission of leukemia was unsuccessfull.