A pure culture of oligodendrocytes has been developed starting from brain hemispheres of newborn rats. Various effects of acidic and basic fibroblast growth factors (FGFs) on the development of oligodendrocytes have been examined and compared. Both factors elicited similar effects, i.e. stimulation of the proliferation, inhibition of the specific activity of the marker enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase and decrease of the ratio of myelin basic protein positive cells. These results indicate that FGFs are very potent mitogens for oligodendrocytes, even in the absence of other cell types, but that they elicit a negative effect on the cell maturation, possibly related to their strong effect on proliferation.