PIP: A sesquiterpene lactone given the trivial name leucanthanolide, from the Mexican zoapatle plant (Montanoa leucantha), long used as an herbal remedy, was isolated and was found to have cytotoxicity but no uterine activity. The compound was isolated from the fraction containing zoapatanol, a substance being investigated for its ability to induce menses, abortion and labor. The compound has a molecular formula of C19H2606, a 5-membered lactone ring, an ester group, and a germacradienolide skeleton. It was evaluated for abortifacient activity in pregnant guinea pigs by intraperitoneal injection on day 22 of gestation. 3 of 5 animals had abnormal fetuses, but there was no evidence of early uterine activity. Cytotoxic testing in vitro was done on KB and P-388 test systems in cell culture. The ED50 of leucanthanolide was 0.57 mcg/ml (KB) and 0.93 (P-388). In comparison, the ethyl acetate fraction had an ED50 of 1.35 mcg/ml and 5.2 mcg/ml respectively.