A new compound, penta-acetyl geniposide [(Ac)5-GP], was obtained from modified extract of Gardenia Fructus (San-jee-chee in Chinese). Spectral studies including ultraviolet (UV), mass (MS), nuclear magnetic resonance (NMR), infrared (IR) and elemental analysis (EA) have suggested the chemical structure of the compound as 1-(beta-D-2',3',4',6'-tetraacetyl- glucopyranosyloxy)-1,4a,5,7a-tetrahydro-7-(acetomethyl)-c yclopentapyran-4-carboxylic acid methyl ester. The antitumor bioactivity of (Ac)5-GP in vitro was studied. It exhibited the activities of reduced plating efficiency and inhibition of DNA synthesis in cultured C-6 glioma cells, but had little effect on RNA and protein synthesis. These results implied that the acetylated iridoid glycoside of Gardenia Fructus is biologically active as an antitumor agent against C-6 glioma cells in culture.