The effects of ursolic acid on the proliferation of B16, a mouse melanoma cell line, were studied. During investigations of the anti-proliferative effects of this triterpene, we observed that MTT colorimetric and colony forming assays show that ursolic acid is a potent inhibitor of B16 cell growth. Cell cycle analysis was performed by propidium iodide staining and flow cytometry technique. This triterpene exerts an early effect on cell cycle at G1, which explains its anti-proliferative activity. These results suggest that alterations in cell cycle phase redistribution of B16, by ursolic acid, may significantly influence the proliferation of B16, melanoma cells.