Amongst the properties associated with the sesquiterpene lactones, their cytotoxicity is probably the most valuable. In this in vitro study cytotoxicity of some sesquiterpene lactones from Eupatorium cannabinum L. and related compounds have been tested on a human small cell lung carcinoma cell line, using the Fast Green dye exclusion assay (FGA). From the results certain conclusions could be drawn about structure activity relationship.Eupatoriopicrin, eupatoriopicrin acetonide, “substance 1” and hiyodorilactone E showed highest cytotoxicity (ID50 1-2 µg/ml) following 1 h incubation. Moieties of the sesquiterpene lactones (α-methylene γ-butyrolactone, eupatolide, angelic and tiglic acid) and related compounds (alantolactone and isoalantolactone) were less or not active at these concentrations.From the results of the experiments it can be concluded that the entire molecule (germacranolide ester) is necessary for optimal cytotoxicity in vitro and that cytotoxicity increases with decreasing hydrophilicity.