Gallic acid is a polyhydroxyphenolic compound which can be found in various natural products. It is recognized to be an excellent free radical scavenger and has been shown to induce apoptosis in lung cancer and leukemia cells. No report has addressed whether gallic acid affects mouse leukemia cells in vivo. In this study, we examined the in vivo effects of gallic acid on leukemia WEHI-3 cells and on macrophage phagocytosis. Gallic acid caused a significant decrease in the weights of the spleens and livers from BALB/c mice. One of the major characteristic of WEHI-3 leukemia is the enlarged spleen in mice after i.p. injection of WEHI-3 cells. Gallic acid did not affect the percentages of CD3, CD11 and CD19 markers but decreased the percentage of Mac-3 in a high-dose (80 mg/kg) treatment while promoting Mac-3 levels in a low-dose (40 mg/kg) treatment. Gallic acid promoted the activity, of macrophage phagocytosis in the white blood cells from peripheral blood mononuclear cells (PBMCs) at 40 and 80 mg/kg treatment doses, but decreased the macrophage phagocytosis in isolated peritoneal cells at the 80 mg/kg dose.