Melanoma cells exhibit, both in vivo and in vitro, intrinsic drug resistance to various chemotherapeutic agents. Cultured human melanoma cells (M14) intrinsically express significant amounts of multidrug resistance-related protein (MRP1) and P-glycoprotein (P-gp) in the Golgi apparatus, but do not express these drug transporters on the plasma membrane. A panel of multidrug resistant (MDR) melanoma cell lines (M14Dx), showing different degrees of resistance to doxorubicin (DOX), were isolated. In M14Dx lines, the appearance of surface P-gp, but not of MRP1 or lung resistance related protein (LRP), occurred in cells grown in the presence of DOX concentrations higher than 60 nM. Furthermore, P-gp levels appeared to be dose-dependent. Flow cytometry, laser scanning confocal microscopy and cytotoxicity studies demonstrated that the activity of the drug extrusion system was related to both surface P-gp expression and resistance to DOX. In conclusion, P-gp, but not MRP1 or LRP, might play a pivotal role in the pharmacologically-induced MDR phenotype of melanoma cells.