Retention of fluorescent probes by normal bone marrow cells (A through C) and Thy-1.1^lowSca-1⁺Lin⁻ HSC (D through F) in the presence or absence of pharmacologic MDR modulators. All plots were derived from a single experiment using 7-week-old donor animals. Shaded histograms indicate staining with probe alone, while open histograms indicate staining in the presence of cyclosporin A as an MDR modulator to block dye efflux. In all cases, cells were incubated with the probe for 20 minutes at 37°C, washed, and incubated in the absence of probe for an additional 20 minutes at 37°C to allow efflux. Cyclosporin A, when used, was present during both stages of the incubation; similar results were obtained using verapamil or reserpine as MDR modulators. Cells subsets were isolated for subsequent analysis (Table 2, Fig 6) by gating on and sorting the dullest and brightest 15% of cells in each profile.