Disorazol A,, a macrocyclic polyketide compound that is produced by the myxobacterium Sorangium cellulosum showed a remarkably high cytostatic activity. It inhibited the proliferation of different cancer cell lines including a multi drug-resistant KB line at low picomolar levels. In presence of disorazol A I, the nuclei of the cells increased in size and the cells often became multinucleate. Low concentrations of disorazol (<100 muM) induced an apoptotic process, characterized by enhanced capase-3 activity and DNA laddering, and abnormal, multipolar mitotic spindles.