21, (G2-guanidine-) 22 exhibited high levels of intracellular fluorescence. Interestingly, the molecules were not evenly distributed within the cytoplasm but appeared as localized spots (Figure ). This may indicate energy-dependent uptake through the endosomal pathway. To elucidate whether the uptake of these molecules is energy dependent, cells treated with (G1-NH2-) 19 were incubated at 4 °C and 37 °C, respectively, and analyzed by flow cytometry. As can be seen in Figure , the fluorescence signal of the compounds was significantly reduced when the incubation was performed at 4 °C. This indicates that the uptake of the molecules is energy dependent and therefore not occurring by simple passive diffusion.