The measurement of functional and phenotypic P-glycoprotein by flow cytometry is suitable for cells in suspension, and is particularly appropriate for blood and bone marrow cells. We describe a functional assay for P-glycoprotein using rhodamine 123, an assay for daunorubicin accumulation, and an assay to measure P-glycoprotein levels using the MRK16 antibody. Our protocols include the use of an anti-CD45 antibody for the identification of leukemic blasts. The protocols described in this chapter were designed for use in studies accompanying UK Medical Research Council Trials of patients with Acute Myeloid Leukaemia and High Risk Myelodysplastic Syndrome (?10‰ blasts). These assays are performed in more than one UK Centre, and hence each assay has been subjected to rigorous reproducibility testing (1 ).