Flow cytometric histograms showing functional incorporation of calcein-AM in A549 (A), in HL60 Pgp (B), and in U937 (C) cell lines with CsA (bold line), with probenecid (dotted line), and without modulator (thin line). For A549 and U937 cell lines, the three histograms are superimposed. Flow cytometric histograms showing functional efflux of fluorescent calcein in A549 (D), in HL60 Pgp (E), and in U937 (F) cell lines with probenecid (bold line) and without probenecid (dotted line). For the U937 cell line, the two histograms are superimposed. Pgp and MRP expressions (measured by flow cytometry) of each cell line are noted. *Controls: autofluorescence of the cells that were not exposed to calcein-AM. †We used the ratio of MDR MoAbs (UIC2 or MRPm6) fluorescence divided by control MoAbs (mouse isotype-matched control MoAbs, IgG2A for UIC2 and IgG1 for MRPm6) fluorescence.