We compared the effectiveness of two flow cytometric methods for the detection of the multidrug resistance (MDR) phenotype. The sensitivity of both methods depended on the ability to discriminate low resistance cells from sensitive ones. Therefore, K562 cells with decreasing vinblastine (VLB) resistance levels were examined, the lowest resistance level being nonmeasurable with a colorimetric MTT assay. The fluorescent drug daunorubicin (DNR) was measured in combination with two modulators of MDR, cyclosporin-A (CsA) and verapamil (Vp) in a functional flow cytometric assay. When compared to sensitive cells, DNR uptake levels at steady state were reduced in all resistant cell lines, except for the lowest resistant cell line. The effect of modulator, CsA, on DNR uptake was seen in all resistant sublines, compared to sensitive cells, except for the lowest resistant cells. In another assay, the P-glycoprotein (Pgp) expression was analysed with monoclonal antibodies, MRK16 and C219. MRK16 was found to be the most sensitive antibody to screen for MDR+ cells, since we could show Pgp hyperexpression in all resistant cells. C219 reactivity became evident in cells possessing resistance factors higher than 5. These results indicate that both the functional assay and the Pgp assay are sensitive to be used for screening of MDR+ cells.