The immunoreactivity in OZ42, a neural cell specific antibody that recognizes premigratory cerebellar granule cells, was examined in early postnatal wild-type and weaver mouse cerebella. We find that the OZ42-positive staining in the external granular layer (EGL) is first seen at postnatal day 1 in the most posterior and ventral aspect of midline cerebellum in the wild-type and heterozygous weaver mouse. By postnatal day 4 strong immunoreactivity is observed in the EGL of all cerebellar lobules. This staining is localized to a band of immunoreactive cells present at the interface of the EGL and the molecular layer (ML). In the homozygous weaver cerebellum, OZ42-positive staining is not seen until postnatal day 3. In the postnatal day 4 weaver cerebellum, immunoreactivity is considerably ligther than in littermate control cerebella, and found throughout the width of the EGL (i.e., not localized to the EGL-ML interface). This study demonstrates that the expression of a specific marker of granule cell development is abnormal in the granule cell population of the homozygous weaver mouse, a population of cells known to be intrinsically affected by the action of this mutant gene. In the light of previous studies, which have shown that the weaver phenotype is identifiable as early as the day of birth, and that the OZ42-antigen may be involved with the development process of axonal growth, it is reasonable to suggest that the weaver mutation results in an abnormality in the ability of granule cells to produce and/or stabilize axons.