The normal-reeler chimera mouse (+/+ in equilibrium with rl/rl) provides an experimental system in which an analysis of the migration of immature neurons in the cerebellum can be accomplished. In the present study, five chimera mice were produced from embryos of the wild-type control (C57Bl/6N) and the reeler mutant mouse (BALB/c) by the aggregation technique. The isozyme pattern of glucosephosphate isomerase (GPI) revealed that the brain tissue in the chimera contained both isozymes of the BALB/c (reeler) and C57Bl/6N (normal) strains, implying that internal mosaicism of the cerebellum truly existed. We found no abnormality in the cerebellum of the chimera mouse: the neuronal and glial subpopulations revealed no difference from those of the control. Such normalization of the cerebellum in the chimera suggests that the disturbance of neuronal migration in the reeler is attributable to an abnormal cell-to-cell interaction between migrating young neurons and the radial glial cells.