Neural cell adhesion molecule L1 consists of two glycoprotein bands of 140 and 200 kdaltons at all developmental stages studied (from birth to adulthood in murine cerebellum and cerebral hemispheres) and in the 4 neurological mouse mutants reeler, weaver, staggerer and Purkinje cell degeneration. In histological sections L1 antigen is detectable at birth in the Purkinje cell layer and fiber tracts in the prospective white matter, but not in the external granular layer. From postnatal day 4 onwards L1 antigen additionally appears in the inner part of the external granular layer, the zone of postmitotic premigratory granule cell neurons. The outer part of the external granular layer remains L1 antigen-negative until it disappears at approximately day 12. From then onwards, the antigen remains prominent in the nascent molecular layer and is less detectable in white matter and internal granular layer, the location of the cell bodies of postmigratory granule cells. The four neurological mouse mutants show development of L1 antigen expression analogous to the normal situation, despite an abnormal cellular architecture. In contrast to the central nervous system. Western blots of adult sciatic nerve show a more complex pattern of L1 immunoreactive bands. L1 antigen is detectable on most, if not all Schwann cells in histological sections of sciatic nerve from 17-day-old embryos. At postnatal day 2, only some Schwann cells appear L1 antigen-positive. From then onwards L1 seems most prominently associated with non-myelinating Schwann cells. In monolayer cultures of neonatal dorsal root ganglia the antigen is observed on the surface of neurons and of some Schwann cells. The mutant, trembler, shows a more immature staining pattern for L1 antigen in adult sciatic nerve.