MRP1 activity was evaluated and compared in 11 cell lines with different levels of MRP1 expression using functional assays of calcein acetoxymethyl ester (calcein-AM), carboxyfluorescein diacetate (CFDA) and Rhodamine 123 (Rh123) in combination with the modulators cyclosporin A (CsA), probenecid and MK571. A good correlation was found between MRP1 expression and the modulatory effect of MK571 on calcein-AM uptake (P = 0.01) and probenecid effect on CFDA uptake (P = 0.02). Additionally, the combined modulatory effect of MK571 and probenecid on CFDA uptake (P < 0.0001) and on calcein-AM uptake (P = 0.0001) were highly significant. No correlation was found between MRP1 expression and the effects of three modulators on Rh123 uptake or efflux. In conclusion, calcein-AM and CFDA uptake assays are the best choices to probe MRP1 activity and combination of two modulators may improve the efficiency of these assays. (C) 2003 Elsevier Ltd. All rights reserved.