Flavonoids are polyphenolic compounds found in various foods of plants. Rutin, one of the flavonoids, had been showed induced apoptosis in cancer cells. There is no available information to address rutin affects murine leukemia cells in vivo. In the present study, we are focused on the in vivo effects of rutin on leukemia WEHI-3 cells. The effects of rutin on WEHI-3 in BALB/c mice in vivo were also examined and the results indicated that rutin decreased the percentage of Mac-3 marker, indicating that the differentiation of the precursor of macrophage and T cells was inhibited. The weights of liver and spleen were decreased from rutin-treated groups compared to the control groups and the results indicated that rutin decreased the weight of these organs. One of the major characteristic of WEHI-3 leukemia is the enlarged spleen in murine after i.p. injection of WEHI-3 cells. After the pathological examination, the function of rutin was observed in the liver and spleen in the mice previously injected with WEHI-3 cells. Rutin promoted the activity of macrophage phagocytosis in cells which isolated from peritoneal (i.p.). Taken together, rutin can affect WEHI-3 cells in vivo.