Multidrug resistance proteins (MRPs) such as MRP1, MRP2 and MRP3 are membrane efflux pumps involved in multidrug resistance and handling organic anions. In the present study, MRP activity was investigated in normal mature leucocytes and CD34-positive hematopoietic cells from peripheral blood using the flow cytometric carboxy-2',7'-dichlorofluorescein (CF) efflux assay. Basal and similar cellular exports of CF, an anionic fluorescent dye substrate for MRP1 and MRP2 transporters, were evidenced in lymphocytes whatever their subsets (CD3, CD4, CD8, CD20 and CD56 cells), in CD14 monocytes and in CD15 granulocytes whereas higher CF efflux was found in CD34 cells. Such outwardly-directed transports of CF were inhibited by known blockers of MRP function such as probenecid whereas the P-glycoprotein modulator verapamil did not alter the retention of the dye in the blood leukocytes. Peripheral mature blood leukocytes were moreover found to express MRP1 mRNAs and MRP1 protein as assessed by Northern-blot and Western-blot analyses, whereas MRP2 and MRP3 transcripts were not present or only at very low levels. Mature leukocytes therefore display basal constitutive MRP-related transport activity regardless of cell lineage and likely related to MRP1 expression whereas higher MRP-related efflux can be detected in peripheral CD34 hematopoietic cells.