The neural cell adhesion molecule TAG-1 has been implicated in the tangential migration of neurons of the caudal medulla and of cortical interneurons. In the former case, protein is expressed by the neurons as they migrate, and blocking its function results in altered and reduced migration in vitro. In the latter case, protein is expressed, in part, by the pathway the interneurons use to reach the cortex, and in vitro experiments propose a role for TAG-1 in this system, as well. However, the in vivo requirement of TAG-1 in these migrations has not been investigated. In this report, we analyze the developmental phenotype of TAG-1-deficient animals in these two migratory systems. We show that mutant mice have smaller lateral reticular nuclei as a result of increased cell death in the superficial migratory stream of the caudal medulla. On the other hand, the absence of TAG-1 does not affect the number, morphology, timing and routes of GABAergic interneurons that migrate from the ganglionic eminences to the cortex. Therefore, TAG-1 function is required for the survival of the neurons of some precerebellar nuclei, while it is not required for cortical interneuron migration in vivo.