The expression of the neural cell adhesion molecule (N-CAM) on cultured murine oligodendrocytes, their precursors, and myelin was examined by indirect immunofluorescence, biosynthetic radiolabeling followed by immunoprecipitation and Western blot analysis, using antibodies specific for various forms of the molecule. In all culture systems studied, whether the oligodendrocytes were cultured as an enriched fraction containing precursor cells or in the presence of astrocytes and neurons, a similar differentiation-stage-related expression of N-CAM was seen. At early developmental stages many tetanus toxin receptor- and A2B5 antigen-positive putative oligodendrocyte precursors with bipolar morphology were seen and found to express N-CAM in its embryonic form. Of the 04 antigen-positive immature oligodendrocytes with few slender processes most expressed N-CAM, but few the embryonic form of N-CAM. The more mature 01 or 010 antigen-positive oligodendrocytes were found to express exclusively the adult form of N-CAM. Oligodendrocytes synthesized the 120 and 140 kD forms of N-CAM (N-CAM 120 and N-CAM 140), but not N-CAM 180, although with differentiation, N-CAM 120 predominated in oligodendrocytes and also in pure myelin. N-CAM 120 could be released from oligodendrocytes and myelin by phosphatidylinositol-specific phospholipase C, suggesting that in both oligodendrocytes and myelin N-CAM 120 is inserted into the membrane by covalent linkage to phosphatidylinositol.