A routine screening of randomly selected native plants for biological activity revealed that an extract from Chelidonium majus elicited significant cytotoxicity, but was devoid of antitumor properties. A subsequent investigation of the alkaloids from this plant resulted in the isolation of coptisine as the chloride. Coptisine chloride and a second new alkaloid have been shown to be two of the cytotoxic principles of Chelidonium majus. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company