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Four new triterpenoids, 6beta-hydroxy-3-oxo-11alpha,12alpha-epoxyolean-28,13beta-olide (1), 3beta,6beta-dihydroxy-11alpha,12alpha-epoxyolean-28,13beta-olide (2), 3beta,6beta-dihydroxy-11-oxo-olean-12-en-28-oic acid (3), and 3beta-hydroxy-12-oxo-13Halpha-olean-28,19beta-olide (4), and five known triterpenes, 19alpha-hydroxy-3-oxo-olean-12-en-28-oic acid (5), 6beta-hydroxy-3-oxo-olean-12-en-28-oic acid (6), sumaresinolic acid (7), siaresinolic acid (8), and oleanolic acid (9), were isolated from the resin of Styrax tonkinensis. The structures of these triterpenoids were determined by physicochemical and spectroscopic methods. The configuration of compound 4 was confirmed by X-ray crystallographic analysis. All these triterpenoids inhibited HL-60 cell growth with IG(50) values ranging from 8.9 to 99.4 microM. Oleanolic acid (9) was the most effective antiproliferative agent, with an IG(50) value of 8.9 microM. While 3beta,6beta-dihydroxy-11-oxo-olean-12-en-28-oic acid (3) exhibited the least effective growth inhibition among these triterpenoids, it induced HL-60 cells to undergo differentiation as measured by an NBT reduction assay.