In the developing mouse hindbrain, immunoreactivity for calretinin, a calcium-binding protein, was first observed at embryonic day 10, and was localized to neuronal cell bodies in the reticular formation. By embryonic day 12, fibers emanated rostrally from the calretinin-immunoreactive neurons, extended dorsally and then caudally in the uncinate fasciculus to reach the developing cerebellar plate. These fibers crossed the cerebellar midline and were distributed to the contralateral side of the cerebellum. The number and intensity of staining of cell bodies in the reticular formation was reduced in postnatal mice. After postnatal day 1, it was no longer possible to discern the calretinin-immunoreactive fiber bundle in the brainstem, although fibers were still visible at the level of the uncinate fasciculus and in the cerebellum. We also observed intensely calretinin-immunoreactive, smaller cells in the cerebellum (embryonic day 14) and dorsal cochlear nuclei (embryonic day 18), most of which we believe are destined to become the "unipolar brush", (also known as "pale" or "monodendritic") cells observed in the adult mammalian brain. An immature form of these cells exists in the developing mouse cerebellum. Thus, using calretinin antiserum as a marker, an afferent neuronal system was described which projects to the cerebellar primordium. It is suggested that the calretinin-containing hook bundle is an afferent projection which provides a feed-forward neuronal system to the cerebellum which, in turn, projects afferent fibers to the calretinin-containing and other cells of the reticular formation.