A rat monoclonal antibody (OZ42), raised against immature mouse granule cells, recognizes a region of the external granular layer of postnatally developing cerebellar cortex. This region, about three cells thick, is adjacent to the developing molecular layer and contains postmitotic, premigratory granule cells. The OZ42 reactivity commenced near postnatal day 3 (P3), the deep external granular layer was strongly reactive by P10 and this level was maintained while granule cells remained in the external granular layer (approximately P15). Isolated immature granule cells in cytospin preparations specifically reacted with OZ42. Reactivity was extranuclear and was substantially reduced when cells were prepared by trypsinization, suggesting that at least some of the antigen is associated with the outer surface of the plasma membrane. Other postnatal reactivity to OZ42 (P0 to P3) was found in a band of cells in the deep cortical layers overlying the corpus callosum through the entorhinal cortex, terminating adjacent to the hippocampus. Reactivity in some regions of the corpus callosum and anterior commissure was seen from P0 to P5. No reactivity of non-neural tissues was observed at any stage. In the embryo there was extensive staining of the CNS and PNS at E10 and E14, which was largely gone by E16. Weaver mutant mice examined for reactivity to OZ42 showed that the granule cell death and cerebellar disorganization in P10 homozygous mutants was associated with a substantial decrease in OZ42 reactivity in the external granular layer. At P14 and P20, OZ42 reactivity in the weaver external granular layer was restricted to single cells, rather than an entire layer of cells, further indicating that the OZ42 antigen is present on granule cells rather than the substratum. By Western analysis of non-reducing SDS-PAGE gels, OZ42 recognized a single band with the molecular weight between 120 and 145 kD in P10, but not adult cerebellum and BALB/c mice. An OZ42-specific band at 60-70 kD was also seen under reducing conditions and occasionally in non-reducing conditions. These bands were not recognized by antibodies against NCAM, L1 and AMOG. Immunoprecipitation and cross-blocking with antiserum to TAG-1 suggested that OZ42 recognized the same molecule in the mouse cerebellum that has been described in embryonic rat and mouse spinal cord. The developmentally regulated expression of the neural-specific molecule recognized by OZ42 in the postnatal cerebellum suggests it my be involved with the early stages of granule cell axon elongation.