Abstract A soluble form of the D2 glycoprotein, detected in the rat brain hypotonic extract, is described. Its specific relative concentration did not differ significantly in the three examined cerebral regions (forebrain, brainstem and cerebellum), while in the cerebellum the membrane-bound form was about three and four times more concentrated than in the forebrain and brainstem, respectively. No sizeable developmental variations of the soluble D2 concentration could be detected in forebrain, whereas the amount of the membrane-bound protein rose from birth to postnatal day 6 and then decreased to the adult value (about 40% of the newborn concentration). Ontogenetic modifications of the membranous D2 glycans (studied through the binding of the molecule to several lectins) occur around postnatal day 18 when the binding to Ricinus communis lectin, specific for galactose, becomes evident. At all ages both soluble and membrane-bound forms bind to Concanavalin A, specific for mannose and glucose, and to wheat germ agglutinin, specific for N-acetylglucosamine, while the lack of binding to Ulex europeus lectin suggests the absence of discrete amount of fucose. The results are discussed in relation to the possible involvement of D2 glycoprotein in cell-to-cell adhesion.