In the previous experiment, the author succeeded in isolating viruses of the original lymphocytic leukemia and the changed myclogenous leukemia in C58 and AKR mice. In the present experiment, the author performed cross neutralization tests between these viruses according to the method of Hamazaki. Though coss neutralization in C58 leukemias were generally observed, strictly speaking, antigenicity of the original and variant viruses were not completely identical. The author obtained the same results with the AKR leukemias. The viruses of the original lymphocytic leukemia and the changed myelogenous leukemia were found to be immunologically closely related and a change of the leukemia type was considered to be due to the difference in the tissue affinity of these two different viruses originally co existed.