By a serendipitous event, a crude fraction of podophyllin was treated with benzaldehyde, resulting in a mixture of products that were mainly benzyl derivatives of lignin glycosides (Stahelin & von Wartburg 1991). The crude reaction mixture was highly cytotoxic and active in the mouse life-prolongation assay against a leukaemia cell line. Some of the components of this mixture were isolated but were not as active as the whole mixture. Condensation of benzaldehyde with the reaction mixture generated a series of acetals that were not hydrolysed by glucosidase enzymes and were less water-soluble. Additionally, the crude mixture worked by a different mechanism from the purified products by stopping the tumour cells from the undergoing mitosis. This preparation was marketed for cancer treatment under the name of Proresid. Work on Proresid with bioassayguided isolation against a mouse life-prolongation assay with a leukaemia cell line indicated that there was a highly active agent present. As with paclitaxel and camptothecin, in the 1960s it was very difficult to isolate and identify minor components and the process took several years to complete. The most abundant component of Proresid is podophyllotoxin benzylidene glucoside; a minor component is 4’-demethylepipodophyllotoxin benzylidene glucoside, which was found to be the most active agent.