The expression of several cell type-specific markers was studied by indirect immunofluorescence in discontinuous BSA gradient fractionated and unfractionated mouse cerebellar cells cultured for 3 days in vitro from embryonic day 13 through postnatal day 9. Cell surface antigen NS-4 and tetanus toxin receptors are present at all ages studied. Thy-1 first detected on neurons with large cell bodies (Purkinje and/or Golgi Type II.neurons) onpostnatal day 3, but absent from all neurons with small cell bodies (granule, basket, and stellate cells). At all ages Thy-1 antigen is absent from astro- and oligodendroglia, and fibroblasts or fibroblast-like cells. Fibroblast-like cells express fibronectin at all ages studied. Astroglia expressing glial fibrillary acidic (GFA) protein are first detectable in cultures from 16-day-old embryos. Their number increases at later ages. At all ages studied fluorescein conjugated Ricinus communis agglutinin 120 brightly labels the cell surfaces of fibroblastic cells and large neurons, less brightly those of astrocytes, bu not those of small neurons. Oligodendroglia become detectable in cerebellar cultures from 16- and 17-day-old embryos maintained in vitro for 3 days using antibodies to 04 antigen and bovine corpus callosum, respectively. At embryonic ages BSA step gradient procedures do not result in enrichment of particular cell types as recognized by the available markers. From birth onward, however, enrichment of cell populations was obtained corresponding to the ones characterized at postnatal day 6 as described in the companion paper (Schnitzer and Schachner 1981b).