Lantadene C (22 beta-2-methylbutanoyloxy-3-oxoolean-12-en-28-oic acid) isolated from the leaves of the hepatotoxic plant Lantana camara var. aculeata (Red) has been found to be identical with dihydrolantadene A reported earlier. Molecular structure of lantadene C has been deduced from single crystal X-ray diffraction analysis. It resembles lantadene A in the pentacyclic portion of the molecule but differs in the side chain region. Atom C-34 is cis to C-35 in lantadene C but is trans in lantadene A. Semisynthetic lantadene C was prepared by catalytic hydrogenation of lantadene A. Lantadene C was obtained in two forms, I and II. Form I was crystalline while form II was amorphous. Unlike lantadene A, both form I and II of lantadene C elicited strong hepatotoxic response in guinea pigs associated with decrease in fecal output, feed intake, hepatomegaly, hepatic injury at the cellular and subcellular level, increase in plasma bilirubin, and acid phosphatase activity. All the clinical signs, hepatic lesions, and changes in blood plasma typified lantana toxicity. This is the first report on the hepatoxicity of lantadene C. The interrelation of molecular structure and biological activity of lantadene A and C has been discussed.