Cucurbiturils (CBs) of the appropriate size (CB[7] and CB[8]) form strong guest-host complexes in phosphate buffer solution (PBS) with acridine orange (AO) and pyronine Y (PYY) with 1 : 1 and 2 : 1 stoichiometries for CB[7] and CB[8] complexes, respectively. Binding constants in the range 0.87-1.60 x 10(6) M(-1) and 5.2-6.3 x 10(13) M(-2) were determined by titration with fluorescence spectroscopy for 1 : 1 and 2 : 1 complexes, respectively. These binding constants in PBS and the eight-fold excess of CBs minimize the presence of free dye in solution and also stabilize the host-guest complex in the culture medium. Images showing that the CB complexes can cross the cell membrane of 3T3 cells have been acquired using fluorescence microscopy. Given the current importance of supramolecular CB complexes and the search for new drug delivery systems, the present findings open avenues for the use of CBs as nanocapsules to transport drugs into the cells.