Many studies have established the central involvement of the Golgi apparatus in the transport and processing of plasma membrane, lysosomal, and secreted proteins. The Golgi apparatus of neurons is also involved in the axoplasmic flow of fast-moving macromolecules and in the orthograde, retrograde, and transsynaptic transport of exogenous ligands. Markers of the Golgi apparatus, based on traditional methods of enzyme cytochemistry, are not applicable to human tissues obtained at autopsy. For that reason, the Golgi apparatus of brain cells has not been examined adequately in diseases of the human nervous system. Here we report that an antiserum raised against MG-160, a 160-kDa sialoglycoprotein of medial cisternae of the Golgi apparatus of several rat cells, is a specific and easily reproducible immunocytochemical marker of the Golgi apparatus of human neurons and other cells obtained at autopsy. Application of this probe in amyotrophic lateral sclerosis has shown a fragmentation of the Golgi apparatus in motor neurons similar to that induced by depolymerization of microtubules. We suggest that the fragmentation of the Golgi apparatus of motor neurons in amyotrophic lateral sclerosis has functional implications because significant reductions of secretion of insulin and immunoglobulins have been observed in islet cells and plasma cells, respectively, treated with microtubule-disrupting agents.