Disclosed is a system for detecting a protease inside a cell. In one embodiment, the system includes a chimeric protein that comprises as covalently linked components: 1) at least one optionally masked signal protein; 2) at least one protease-specific cleavage site; and 3) at least one detectable amino acid sequence. The invention has a wide spectrum of applications including use in the detection of novel protease inhibitors inside cells and tissue.