The differentiation of bipotential precursors of oligodendrocytes (OL) and type-2 astrocytes (AS) was followed in primary cultures from 8-day postnatal rat cerebellum by labeling the cells with the antibodies LB1 (which binds to the surface disialoganglioside GD3 present in glial precur- "neuron-like" fashion (6,11), are sensitive to kainic acid (V. Gallo, R. Suergiu, C. Giovannini, and G.L., unpublished data), and express chondroitin sulfate (7). The bipotential cerebellar glial precursors pass through four distinct developmental stages during their differentiation into OL in serum-free cerebellar cultures (9). In the first stage, the cells are A2B5+ and LB1'. In the second stage, they can be stained also by the mAb 04, which binds to surface sulfatides and recognizes both immature and mature OL (12). In the third, short-lasting stage, the cells express on their surface also the well-established marker of OL, GalCer (13). In the fourth stage, however, the antigens recognized by A2B5 and LB1 are no longer detectable, and the cells remain positive for 04 and GalCer. Upon addition of FCS, the precursors maintain their ability to differentiate into type-2 AS until they are in the A2B5/LB1/04-positive stage (9). The factors responsible for the choice of the differentiation route taken by the precursor cells have not been defined. It (4,6) were resuspended in Eagle's basal medium (BME, GIBCO) supplemented with 2 mM glutamine, gentamycin (Sigma) at 0.1 mg/ml, and 10% heat-inactivated FCS (different batches from GIBCO); were seeded at 1 x 105 (low density) or 2.5 x 105 (high density) cells per cm2 onto