This light and electron microscopic immunocytochemical study shows that the polypeptide PEP-19, a presumptive calcium binding protein specific to the nervous system, represents an excellent marker for cerebellar Purkinje cells and dorsal cochlear nucleus (DCoN) cartwheel cells. The polypeptide clearly reveals the entire populations of both types of neurons, including their complete dendritic and axonal arborizations. Other PEP-19 containing neurons in the two regions display weak immunoreactivity restricted to the cell body or to cell body and principal dendrites. Electron microscopic localization of PEP-19-like immunoreactivity reveals similarities between this polypeptide, parvalbumin, and a 28K vitamin D-dependent calcium binding protein. However, calmodulin, which is expressed in both Purkinje and granule cells, may differ from PEP-19. Similarities between the organization of the cerebellar cortex and the DCoN superficial layers have been known for some time, with several types of neurons in one system having their presumed homologue in the other. These data provide further support for the proposed structural and functional homology between Purkinje and cartwheel neurons, and establishes PEP-19 as a useful marker for examining degeneration of these two neuronal populations in murine cerebellar mutants.