Apoptolidin, originally isolated from a Nocardiopsis sp., induces apoptotic cell death in rat glial cells transformed with the adenovirus E1A oncogene (IC50 = 11 ng/ml). Apoptolidin is among the most selective cytotoxic agents tested by the NCI in human cancer cell lines. Although the apoptotic activity of apoptolidin correlates with F0F1-ATPase inhibition, recent evidence suggests the existence of a secondary biological target or more complex mode of action.