Acetylarsonate was prepared for testing antitumor and immunological effects. It showed cytotoxicity directly on Sarcoma 180. L1210 and MOLT-4 by MTT assay. It did not seemed to trigger the mitosis of human lymphocytes in culture, but that showed the cytotoxicity with higher dose. The rosette formation and spleen weight of mouse which acetylarsonate was administered to for 2 weeks were increased. Furthermore, peripheral helper T- and cytotoxic/suppressor T-lymphocytes were increased in acetylarsonate-injected-mice significantly when it was estimated with simultaneous 2 color analysis using anti Lyt2-FITC and L3T4-PE monoclonal antibody by Flow cytometer.