Fluorescence tomography today appears as a complementary imaging modality for assessing molecular processes in small living animals. A new design of fluorescent activatable units for the imaging of cellular internalization of probes is proposed. Disulfide bridges are used as intracellular cleavable bounds for fluorescence activation. These units are not activated in blood, neither in vitro nor in mouse, but specifically in the presence of lysed cells or a chemical reducer. We therefore expect that the use of such activatable units, grafted to targeting moieties, will improve significantly the contrast of images obtained in the future.