Wedelia asperrima Benth. (Compositae) causes serious sheep mortality in northwestern Queensland. Earlier work isolated a toxic principle called wedeliatoxin but did not perform structural analysis; we recommend discontinuing this name to avoid confusion. Subsequent studies found higher toxicity in the dialyzable fraction of plant extracts, leading to a new isolation procedure. This study reports the isolation, purification, characterization of the major toxic dialyzable component (named wedeloside) and its antitumor activity against aflatoxin B1-induced tumors in rats. Isolation steps included methanol-water extraction, DEAE-cellulose chromatography, BIO-RAD AG-50 columns, and preparative TLC, yielding 20 mg of pure wedeloside (single TLC spot in two systems, LD50 ~1 mg/kg in mice/rats). Structural analysis using mass spectrometry (permethylated/perdeuteromethylated derivatives), GC-MS, 13C- and 1H-NMR revealed wedeloside’s unusual structure: an amino-sugar glycosidically linked to a diterpene (rare in plants, first reported for such a linkage), a rare phenyl propionate ester on the glycoside, and an unusual kaurene ring oxygenation pattern. A second toxic component (structure 2) with an additional rhamnose moiety was also isolated. For antitumor testing, 10 rats were induced with tumors via weekly aflatoxin B1 doses (total 1 mg/rat). Five rats treated with wedeloside (0.1 LD50 weekly for 8 weeks) had no tumors at autopsy, while all 4 controls had tumors. These results indicate wedeloside inhibits aflatoxin B1-induced tumors in rats. Wedeloside’s structure is notable for its rare amino-sugar-diterpene linkage, unusual ester, and kaurene oxygenation.