We have recently found that about 50% of newly formed oligodendrocytes normally die in the developing rat optic nerve. When purified oligodendrocytes or their precursors are cultured in the absence of serum or added signalling molecules, they die rapidly with the characteristics of programmed cell death. This death is prevented either by the addition of medium conditioned by cultures of their normal neighboring cells in the developing optic nerve, or by the addition of platelet-derived growth factor (PDGF) or insulin-like growth factors (IGFs). Increasing PDGF in the developing optic nerve decreases normal oligodendrocyte death by up to 90% and doubles the number of oligodendrocytes, suggesting that this normally occurring glial cell death might result from a competition for limiting amounts of survival signals. These results suggest that competition for limiting amounts of survival factors is not confined to developing neurons, and raise the possibility that a similar mechanism may be responsible for some naturally occurring cell deaths in nonneural tissues.