Casticin, derived from Fructus Viticis, has anticancer properties in many human cancer cells, however, there is no report to show that casticin promotes immune responses and affects the survival rate of leukemia mice in vivo. The aim of this study is to evaluate the effects of casticin on immune responses and the survival rate of WEHI-3 cells generated in leukemia mice in vivo. Animals were divided into six groups: normal control mice, leukemia control mice, mice treated with ATRA (all-trans retinoic acid), and casticin (0.1, 0.2, and 0.4 $$ mg/kg) treated mice. All animals were treated for 14 days and then measured for body weights, total survival rate, cell markers, the weights of liver and spleen, phagocytosis of spleen cells, NK cell activities and cell proliferation. Results show that casticin did not affect animal appearances, however, it increased body weights and decreased the weights of liver at 0.2 $$ mg/kg and 0.4 $$ mg/kg treatment. Casticin also decreased spleen weight at 0.2 $$ mg/kg and 0.4 $$ mg/kg treatment, increased CD3 at 0.1, 0.2 and 0.4 $$ mg/kg doses and increased CD19 at 0.2 $$ mg/kg treatment but decreased CD11b and Mac-3 at 0.1, 0.2 and 0.4 $$ mg/kg treatment. Casticin (0.1, 0.2 and 0.4 $$ mg/kg) increased macrophage phagocytosis from PBMC (peripheral blood mononuclear cell) and peritoneal cavity. Furthermore, casticin increased NK cells' cytotoxic activity and promoted T cell proliferation at 0.1-0.4 $$ mg/kg treatment with or without concanavalin A (Con A) stimulation, but only increased B cell proliferation at 0.1 mg/kg treatment. Based on these observations, casticin could be used as promoted immune responses in leukemia mice in vivo.Keywords: Casticin; Immune Responses; Leukemia Mice; Macrophage Phagocytosis; NK Cell Activities.