Various agents have been shown to enhance drug sensitivity of multidrug resistant (MDR) cells and are thus of interest when the MDR phenotype is identified. Detection of MDR cells is of importance and can be carried out either by immunofluorescence with monoclonal antibodies or by functional tests using fluorescent dyes uptake. MDR has been analysed by flow cytometry on three sensitive and resistant cell lines, with MRK16 and C219 monoclonal antibodies directed against P-glycoprotein (P-gp) and with rhodamine 123, Hoechst 33342 and daunorubicin. Resistant cells were revealed by MRK16 and C219 but the results obtained with MRK16 gave higher both percentages of fluorescent cells and mean fluorescence. Fluorescence intensity observed with daunorubicin was lower than with rhodamine 123. With Hoechst 33342, mean fluorescence was quite identical on sensitive and on resistant cells. It was concluded that MRK16 and rhodamine 123 were well adapted to detect P-gp and evaluate its functional ability.