Spleen cells from BALB/c mice, previously immunized with rat cerebellar tissue, were fused to the mouse myeloma cell line SP2/0-Ag14 and the cerebellar cell type specificity of the resultant hybridomas determined. In this report we describe the specificity of one hybridoma, C4/12. Monoclonal antibodies secreted by this hybridoma recognize granule cell neurons in adult cerebellar frozen sections, and in primary cultures started from 3 to 5-day-old newborn rats. In addition, C4/12 recognizes a subclass of astrocytes when screened on primary cultures but not adult cerebellar tissue. Two temperature sensitive Rous sarcoma virus transformed cerebellar cell lines, previously shown to be either neuronal or glial, were screened for the presence of the antigen. Both cell lines are positive at the temperature permissive for transformation, whereas the glial line but not the neuronal line exhibits the antigen at the nonpermissive temperature. These results are discussed in light of the cell lines being representative of precursor cells.