1) Myelogenous and lymphocytic leukemias were observed among mice given orally three times a week 0.1-0.15cc of 0.8% 20-methylcholanthrene in olive oil by means of a needle catheter and concurrently painted in the back two times a week with 5% croton oil in benzene. 2) The incident of leukemias was 61.6% for myelogenous and 5.5% for lymphocytic leukemia with a total of 66.6%. On the other hand, mice receiving croton oil alone developed no leukemia. 3) Concurrent administration of 20-methylcholanthrene and croton oil reduced the average latent period required for the development of leukemia by about four weeks in comparison with that of 20-methylcholanthrene treatment alone. 4) From these results, it is concluded that croton oil is coleukemogenic, acting as a promoter in the 20-methylcholanthrene leukemogenesis in mice.