The enlarged vesicles in the lvsB-null cells are acidic lysosomes. To visualize lysosomes, NC4A2 control (A and B), lvsA-null (D and E), and lvsB-null (G and H) cells were subjected to a 15-min pulse with FITC-dextran, washed, and chased for 15 min in fresh growth medium. Cells were examined with phase-contrast optics (A, D, and G) or by fluorescence microscopy (B, E, and H). The control (B) and lvsA-null (E) cells contained lysosomes of normal size, whereas the lvsB-null cells (H) contained enlarged lysosomes. Control (C), lvsA-null (F), and lvsB-null (I) cells were incubated with the acidophilic dye LysoSensor DND-189 (Molecular Probes) in HL-5 growth medium and visualized using a fluorescence microscope. The arrows point to enlarged acidic lysosomes, and the arrowheads point to normal size lysosomes. Bar, 2 μm.