Fluorescent single-wall carbon nanotubes (SWCNTs) were prepared by mixing cut SWCNTs with acridine orange (AO). The optical absorbance and fluorescence characteristics of AO–SWCNT conjugates display interesting pH-dependent properties. Fluorescence microscopy in combination with transmission electron microscopy proves that AO–SWCNTs can enter HeLa cells and are located inside lysosomes. The endocytosis-inhibiting tests show that the clathrin-mediated endocytosis is a key step in the internalization process. The internalized AO–SWCNTs remain inside lysosomes for more than a week and have little effect on cell proliferation. These findings may be useful in understanding the SWCNT-based intracellular drug delivery mechanism and help to develop new intracellular drug transporters.