A major event of nervous system development is the migration of granule cell neurones, during the early postnatal development of the cerebellar cortex, from their germinating zone in the external granular layer to their final location in the internal granular layer. During migration, many granule cells are seen in direct cell-surface contact with processes of Bergmann glia, a subclass of astrocytes. In the neurological mutant mouse weaver, however, migration of granule cells is impaired, probably due to a deficit in cell-cell interactions. To gain insight into the cellular and molecular mechanisms involved in granule cell migration, we have used a modification of an in vitro assay system, previously described by Moonen et al., which displays migratory behaviour in small tissue explants during several days of suspension culture. The aim of this study was to investigate the process of granule cell migration by using antibodies directed against cell-surface components of developing neural cells. We report here that migration of 3H-thymidine-labelled granule cell neurones can be modified by Fab fragments of both mono- and polyclonal L1 antibodies, but not by Fab fragments of polyclonal antibodies prepared against mouse liver membranes, which also react with cerebellar cell surfaces.