Comparison of FR expression on multiple subpopulations of CD34⁺ cells. LDMNC were stained with anti–CD34-APC and either (A) anti–CD38-PE or (B) anti–CD15-PE (20 minutes at 4°C). After washing, the cells were incubated with either rabbit preimmune serum or anti-FR (30 minutes at 4°C), washed, and incubated with the secondary antibody, goat anti-rabbit-FITC (30 minutes at 4°C) before analyzing the cells by three-color fluorescence cytometry. (A) FR expression (FL1 on x-axis) after anti-FR staining is shown for CD34⁺CD38⁻ (light gray) and CD34⁺CD38⁺ (black) cells. CD34⁺ cells that were incubated with preimmune serum are represented by the dark gray peak in both (A and B). These data indicate that a significant proportion of CD34⁺ cells express FR and that CD34⁺CD38⁺ cells have slightly higher levels of FR expression than CD34⁺CD38⁻ cells. (B) FR expression levels on CD34⁺CD15⁻ (black) and CD34⁺CD15⁺ (light gray) cells are shown on this histogram. The results show that the more differentiated CD34⁺CD15⁺ cells have an increased proportion that express FR. In addition, CD34⁺CD15⁺ cells are shifted to the right further than any other population evaluated, suggesting that these cells have an increased number of FR/cell.