The splenic tissue of C58 strain mice with myelogenous or lymphocytic leukemia, and mammary cancer tissue of C3H strain mice were transplanted on the chorio allantoic membrane, in the allantoic cavity and in the yolk sac of embryonated eggs. Observations on the growth of the transplanted tumors and their influences on the embryos have provided the following results. I) Transplantation on the chorio-allantoic membrane. In the case of myelogenous leukemia and mammary cancer, growth of the transplanted tissue and a reactive response of various kinds of host cells in the surrounding chorio-allantoic membrane were observed. While these changes were slight in the ease of lymphocytic leukemia. The transplanted splenic tissue of myelogenous leukemia was successfully back-transplantable after five days' cultivation in the heterologous host. As controls, normal splenic tissue of C58 strain mice were similarly transplanted, but soon regressed in the course of the cultivation, 2) Transplantation in the allantoic cavity. Grossly and histologically, no signficant changes were observed in all cases. 3) Transplantation in the volk sac. The chick embryos died in 100% of cases in the transplantation of myelogenous leukemia and mammary cancer. On the other hand, the death rate was reduced to 50% in the case of lymphocytic leukemia. In the case of mammary cancer some localized necrosis was observed in the liver of the embryos.