Normal cellular homeostasis employs a delicate balance between pro-apoptotic (cell death) and anti-apoptotic (growth-stimulatory) forces. Unrestricted/unregulated cell growth, which occurs in neoplasia frequently, accompanies an imbalance in these forces and frequently the apoptotic pathways are disrupted leading to increased metastatic spread, resistance to chemotherapy and radiotherapy. Pro-apoptotic enzymes such as the serine protease Granzyme B can activate the pro-apoptotic cascade within tumor cells through multiple, independent mechanisms. We have generated novel cell-targeting fusion constructs containing enzymatically active GrB and have demonstrated that these constructs are highly cytotoxic to target cells. Apoptotic cascades can be maximally activated within 4 hr of treatment and these agents are synergistic with chemotherapeutic agents. We propose that these types of signaling agents, which operate exclusively through activation of pro-apoptotic mechanisms, may describe a new class of targeted therapeutic agents with unique biological properties.