The distribution of the large, external, transformation-sensitive (LETS; fibronectin) protein was investigated in rat cerebellum, both in vitro and in vivo, by biochemical and immunocytochemical methods. Biochemical analyses indicated that LETS protein is not demonstrable on the surfaces of cerebellar neurons from postnatal rats maintained in cell culture for varying periods of time, but is present on the surfaces of at least some fraction of the total nonneuronal cell population in vitro. Indirect immunofluorescence microscopy with an anti-LETS antiserum substantiated these observations and further indicated that LETS-bearing cells of cerebellum maintained in vitro are probably of endothelial and fibroblastic origin. The LETS protein is arranged in a reticular network of filaments spanning the surfaces of the cells, and the filaments are often extensively interdigitated with each other. At all stages of development investigated (two days postnatal to adult) LETS antigen was observed in vivo to be primarily localized in the meninges covering the surface of the cerebellum and between folia, and in the walls of blood vessels within the tissue. Neuroblasts and neurons of the external and internal granule layers of the cerebellum, respectively, were negative for the presence of LETS antigen.