The serum proteins were investigated by cellulose acetatate electrophoresis in Rauscher leukemic mice and leukemic mice which were either induced or not induced to remission with virus vaccines. In leukemic mice serum γ-globulin level increased. slightly as the mice approached the initial stage of leukemia. In the abvanced stage of leukemia there were a decrease of γ-globulin, α-globulin and albumin and an increase of β-globulin. Twenty days after virus inoculation, an abnormal fraction, not found in normal mice, was detected between α(2)-globulin and β-globulin fractions and markedly increased as leukemia prolonged. This abnormal fraction was considered to be specific in Rauscher leukemia. In mice which were induced to remission with vaccines, the abnormal fraction disappeared in the remission stage and there were a marked increase of γ-globulin and β-globulin probably due to reaction of the host against Rauscher leukemia. And the abnormal fraction appeared again in mice which relapsed.