Berberine, an isoquinoline alkaloid, has a wide range of pharmacological effects including anticancer activities, yet the exact effects on leukemia in vivo are unknown. Our previous studies have demonstrated that berberine induced cytotoxicity against murine leukemia WEHI-3 cells in vitro in a dose-dependent manner. In order to understand the berberine action against leukemia, the effect of berberine on WEHI-3 leukemia cells in vivo was studied. The results showed that Mac-3 and CD11b markers were reduced, indicating differentiation inhibition of the macrophages and granulocytes precursors. There was no affect on the CD14 marker but the CD19 marker that was indicating the promotion of the differentiation of the B-cells precursors. The weights of spleen samples from mice treated with berberine were found to be lower when compared to these from untreated animals.