Mechanisms of growth inhibition by the novel marine compound dehydrothyrsiferol (DHT) were investigated in a sensitive and an MDR+ human epidermoid cancer cell line. DHT was found to circumvent multidrug resistance mediated by P-glycoprotein. Cell cycle analysis revealed an accumulation in S-phase. The anchorage independent clonogenic growth in soft agar was not significantly reduced at IC50 concentrations. Reduced cell growth caused by induction of apoptotic or necrotic cell death could not be verified. Therefore, cell proliferation during an incubation period of five days was measured and found to be significantly reduced. We conclude that growth inhibition by dehydrothyrsiferol in KB cancer cells is not mediated by apoptosis but by growth retardation; the reasons for this are worth being investigated in detail.