Ursolic acid (UA) is a pentacyclic triterpenoid compound that is widely distributed in the plant kingdom and has a broad range of biological effects. Here, we examined the effects of UA on the proliferation and differentiation of human tumor cell lines from melanoma (A375), glioblastoma (U87) and thyroid anaplastic carcinoma (ARO), and on the proliferation of a non-transformed human fibroblast cell line (WI-38). The results show that UA inhibits tumor cell proliferation in a dose- and time-dependent manner. Consistent with this finding, UA treatment promotes differentiation of all of the analyzed tumor cell lines. Interestingly, we found that UA inhibits the endogenous reverse transcriptase (RT) activity in tumor cells, which has recently been shown to be involved in the control of proliferation and differentiation of neoplastic cells. Considering these findings, we suggest that the observed anti-proliferative and differentiating effects of UA may be related to this target.