The aim of the present study was to understand the role of cellular interactions in the choice of the oligodendroglial differentiation route by bipotential progenitors (O-2A progenitors) of oligodendrocytes (OL) and type-2 astrocytes (AS). Cell populations enriched in 0-2A progenitors, obtained from mixed rat cortical glial cultures, were subcultured at low density in BME+10% fetal calf serum on a poly-L-lysine (PLL) substrate (controls) or on a substrate of purified type-1 AS killed by air drying (K-AS), in order to study the interactions between the two cell types independently of the soluble mitogen(s) secreted by type-1 AS. While on PLL the progenitors differentiated into type-2 AS within a week, on K-AS many of them differentiated into OL. However, on K-AS O-2A progenitors proliferated more than on PLL, and this proliferation appeared to be related to unknown polypeptide components of the extracellular matrix produced by type-1 AS. The preferential differentiation of O-2A progenitors into OL on K-AS may thus be partly related to the higher density achieved by the cells on this substrate. This hypothesis is supported by the observation that purified O-2A progenitors subcultured at high density on PLL largely differentiated into OL. The effect of cell density on lineage decision appeared to be related to the secretion of high molecular weight autocrine differentiation factors by O-2A lineage cells.