With the purpose investigating the possible relation between tissue distribution of methylcholanthrene and leukemogenesis, the author measured quantitatively tissue distribution and urinary excretion of methylcholanthrene after oral administration of the carcinogen. Ⅰ) Urinary excretion of methylcholanthrene: The average urinary excretion of methylcholanthrene at every 6, 12, 18, 24, 30 and 36 hours after one oral administration of 0.8% methylcholanthrene olive oil solution was 6.32, 1.95, 1.15, 0.823, 0.415 and 0.153γ, respectively. Ⅱ) Tissue distribution of methylcholanthrene: 0.15 cc of 0.8% methylcholanthrene olive oil solution was administrated orally three times a week. 1) In bone marrow, methylcholanthrene markedly accumulated in comparison mith other tissues from early time of administration. And in thymus and lymph nodes distribution of methylcholanthrene was clearly recognized from about 8 weeks of administration, but not so much as in bone marrow. 2) At 2, 4, 6, 8 and 12 weeks' repeated oral administration, the average methylcholanthrene in bone marrow was 41.9, 67.0, 108, 118 and 238 γ/g respectively. These figures show a tendency of methylcholanthrene accumulation in bone marrow from the second week. Methylcholanthrene in bone marrow increased in proportion to the number of its administration. 3) At 8 weeks' administration the carcinogen detected was 42.1 γ/g in thymus, and 12.4 γ/g in lymph nodes. At 12 weeks' administration its concentration was 83.8 γ/g in thymus, and 38.6 γ/g in lymph nodes. 4) Nuclear and cytoplasmic fractionation studies indicated a much higher concentration of methylcholanthrene in the nuclear fraction of liver and thymus. From these findings, the author wishes to point out a close connection between leukemogenesis of RF mice by oral administration of methylcholanthrene and its tissue distribution in the hematopoietic organs.