2-(p-Hydroxyanilino)-4-(p-chlorophenyl)thiazole (SK1-II) is a cell permeable potent and specific inhibitor of sphingosine kinase (IC50 = 0.5 μM). The compound does not bind to the ATP-binding site and does not affect the kinase activities of hERK2 hPI3K or hPKCα at concentrations up to 60 μM. SK1-II showed anti-tumor properties inducing apoptosis in a number of cell lines including those that express Pgp or MRP1 drug-transport proteins.