MDR1-associated P-glycoproteindependent multidrug resistance is a common cause of chemotherapy failure in patients with advanced ovarian cancer. Here, we describe a clinical method for simultaneously assessing the expression and function of the MDR1/Pgp in tumour cells from ascites of patients with malignant ovarian carcinoma. Cells from ascites from 35 patients were collected. The expression and function of Pgp were detected by flow cytometry. For functional study, rhodamine 123 was used. Using the Pgp- specific UIC2 and MM6.15 antibodies, we demonstrated the presence of Pgp in 10-79 % (38.9±20, 7; n=35) of the CA125-positive cell subpopulations. The results of the functional assay showed strong correlation with the level of Pgp expression (r=0.976; p=3.2×10^-5). Direct detection of the expression level and function of MDR1/Pgp in the ascites provide useful information for the more efficient treatment of malignant diseases by proper adjustment of the chemotherapeutic protocol.