With the purpose of elucidating the action of 20-methylcholanthrene in leukemogenesis in RF mice by painting of 20-methylcholanthrene, the author measured quantitatively the distribution of 20-methylcholanthrene in the organs of mice after repeated painting. The state of excretion of 20-methylcholanthrene in urine after painting was also investigated quantitatively. Ⅰ) Urinary excretion of 20-methylcholanthrene after painting: 0.2 cc of 0.5% benzene solution of 20-methylcholanthrene was painted at the back of mice. The average urinary excretion at 6, 12, 18, 24, 30, 36 hours after painting was 4.16, 2.35, 0.876, 0.360 0.370, and 0.153γ respectively. Ⅱ) Distribution of 20-methylcholanthrene in the organs of mice after repeated painting: 0.2 cc of 0.5% benzene solution of 20-methylcholanthrene was painted at the back of mice twice a week and organ methylcholanthrene was measured quantitatively at 2, 4, 6, 12 and 16 weeks after repeated painting. 1) After 2 week painting, organ methylcholanthrene was too small to be measured in amount, but tended to accumulate in bone marrow. The average tissue distribution of methylcholallthrene after 6 week painting was 137.6 γ/g in bone marrow. 113.4 γ/g in thymus, 61.2 γ/g in lymph nodes, 39.4 γ/g in spleen, 15.3 γ/g in kidney, 14.5 γ/g in heart, 10.3 γ/g in salivary gland and 12.3 γ/g in stomach. 2) After 8 week painting there was a marked increase of methylcholanthrene in bone marrow and thymus, and after 10 and 12 week painting, it further increased. After 16 week painting average tissue distribution of methylcholanthrene was 518 γ/g in bone marrow, 239 γ/g in thymus, 29.7 γ/g in spleen, 44.9 γ/g in lymph nodes, 18.6 γ/g in kidney, 19.7 γ/g in stomach, 19.8 γ/g in salivary gland, and 10.4 γ/g in heart. 3) The state of tissue distribution of methylcholanthrene in C(3)H mice (resistant to leukemia) was almost the same as compared with that of RF mice. From these findings, it was observed that methylcholantrene, when repeatedly, painted in mice accumulated in a higher concentration in bone marrow and thymus than other organs, suggesting an intimate relationship of the carcinogen and target tissues.