Five new derivatives of the pentacyclic triterpenoid lantadene A (= 22beta-angeloyloxy-3-oxoolean-12-en-28-oic acid; 1) from the leaves of Lantana camara L. were synthesized, characterized, and screened for their cytotoxicities against four human cancer cell lines. The three most-potent compounds, i.e., 1, 4, and 6, with IC50 values in the range of ca. 20-29 microM, were further studied for their in vivo tumor-inhibitory potential upon oral administration in two-stage squamous cell carcinogenesis, using female Swiss albino mice, papilloma being induced by 7,12-dimethylbenz[a]anthracene (DMBA), and promoted by 12-O-tetradecanoylphorbol-13-acetate (TPA). The results are discussed in terms of structure-activity relationship.