Abstract: A method is described to determine multidrug resistance in living cells by laser scanning confocal fluorescence microscopy of doxorubicin in sensitive and resistant tumor cells. The intracellular distribution of doxorubicin in K562 cells has been studied by laser scanning confocal fluorescence microscopy. A different fluorescence pattern has been observed in sensitive and resistant cells. An intense fluorescence signal is evident on the plasma membrane of K562R cells with multidrug resistance. The fluorescence imaging of the drug offers then a new method--non destructive--to discriminate between sensitive and resistant cells, with high potential in the diagnosis of resistance in tumors in vivo. The differences in the intracellular drug distribution, as seen by laser scanning confocal fluorescence microscopy, moreover offer new indications on the mode of action of the drug in the living cell.