Wedelia asperrima Benth. (Compositae) causes acute poisoning and death in sheep within 24 hours after ingestion in northwestern Queensland, with a large amount of straw-colored fluid in the abdominal cavity and lungs being a marked autopsy feature. Earlier, Oelrichs and Muller isolated a toxic principle named wedeliatoxin from the non-dialyzable fraction of its extract, which was highly toxic to laboratory animals and sheep, but no significant structural work was done, so the name is no longer used in this study to avoid confusion. Later investigations found a higher proportion of toxicity in the dialyzable fraction, leading to the development of a new procedure to isolate the toxic components. The major toxic dialyzable compound was isolated and characterized, named wedeloside, with a smallest LD50 dose of approximately 1 mg/kg in mice and rats. Structural elucidation used mass spectrometry of its permethylated and perdeuteromethylated derivatives, combined GC-MS of its derivatized hydrolysis products, and 13C- and 1H-NMR spectrometry by comparison with synthetic and naturally occurring model compounds. Wedeloside has an unusual structure: it is one of the rare amino-sugars in plants other than glycoproteins, the first reported amino-sugar glycosidically linked to a diterpene, with a rare phenyl propionate ester grouping on the glycosidic moiety, and an unusual oxygenation pattern on the kaurene ring of the diterpene aglycone. Tests for antitumor activity showed that among 10 rats with aflatoxin B1-induced tumors, 5 rats treated with wedeloside (0.1 LD50, 30 µg weekly for 8 weeks) showed no tumors on autopsy, while 4 control rats all had tumors. Preliminary experiments suggest wedeloside inhibits aflatoxin B1-induced tumors in rats. Further studies are in progress to determine its cellular effects and impact on mitochondria, as structurally related carboxyatractyloside inhibits oxidative phosphorylation in mitochondria.