VP 16-213, a derivative of podophyllotoxin, is currently entering phase-III studies. Its mode of action is incompletely understood, but differs markedly from that of its parent compound. The greatest lethal damage is experienced by cells in the late S and G2 phases. In the L 1210 system the drug shows marked schedule dependency: prolonged administration may be more effective than single bolus administration. As a single agent, VP 16-213 is the most active compound yet tested against small-cell bronchial carcinoma. It may also prove to be a useful agent in patients with other types of lung tumour, testicular teratomas, and some types of leukaemia. No long-term or cumulative toxicity has been reported. Most side effects are predictable and reproducible.