In this paper is reported a case of acute biphenotypic leukemia who was treated by chemotherapy and pursued its effect by two color flow cytometry. A 33-year-old male patient was admitted due to fever and general fatigue and diagnosed as acute leukemia by hematological findings. Surface markers were investigated to find positive reaction of Leu 12 (CD19), J 5 (CD10), My 7 (CD13) and My 9 (CD33), in which Leu 12 and My 9 were simultaneously expressed on the same blast cells by two color flow cytometry. He was treated with daunorubicin, enocitabine, mercaptopurine, vincristine, and prednisolone to obtain partial remission. Then, he was administered L-asparaginase, doxorubicin, vincristine and prednisolone to reach complete remission. The effect of chemotherapy was investigated by not only bone marrow puncture, but also by two color flow cytometry. From the findings in this case, the two flow cytometry was proved to be a useful tool for not only diagnosis of acute mixed leukemia, aut also the judgement of the effect of treatment.