In 1961 two strains of transplantable leukemia in C58 mice were established in the Department of Internal Medicine, Okayama University Medical School. One is a lymphocytic leukemia (OHS-LL) of spontaneous origin and the other a myelogenous leukemia (OHS-ML) developing in a mouse inoculated with a cell-free extract from the lymphocytic leukemia. The present paper concerns transplantation experiments of both strains of leukemia into isologous mice. Both strains of leukemia were transplantable in 100% , and the age or sex of the recipient mice did not considerably influence the survival days of the animals. The number of the inoculated leukemic cells and life span of the recipient mice showed an inverse relation, and even a single leukemic cell, when transplanted intraperitoneally, killed animals in 18 days. Transplantation of different organs indicated that the number of leukemic cells contained in the organs determined the survival pericds of the inoculated mice, and it was possible to assess the degree of leukemic infiltration in a given organ from the life span of the animals. The site of cell-graft influenced the survival days but very slightly the manifestation of the disease. However, the local changes following intracerebral, intratesticular and subcutaneous transplantation were more pronounced in OHS-LL in comparison with OHS-ML.