During apoptosis, activation of a family of cysteine proteases related to interleukin-1beta-converting enzyme (ICE)-related proteases or "caspases" results in endoproteolytic cleavage of multiple substrates at specific aspartate residues. We have sought to develop new antibody probes for the neoepitopes in protein fragments produced by ICE-related proteolytic cleavage as specific markers of events tightly linked to apoptotic mechanisms. Here, we demonstrate that an antibody probe specific for the C terminus of a 32-kd actin fragment produced by ICE-like activity specifically labels apoptotic but not necrotic, differentiated human neuroblastoma cells in culture. Unlike probes for nonspecific DNA strand breaks confined to the nucleus or cell body, this method allows the detection of cytoskeletal fragments in cell processes as well as the perikaryon long before DNA fragmentation and cell death and therefore serves as a novel marker of apoptosis-related events in distal parts of cells such as axons and dendrites. To illustrate this new tool, we show that the antibody detects the processes and cell bodies of degenerating neurons and plaque-associated microglia in Alzheimer's disease. In situ detection of caspase-cleaved actin provides a new means to evaluate the role of caspase activation in pathological and physiological processes.