alpha-Modified minimal essential medium (alpha-MEM) is the most useful commercial medium for survival and myelin formation of dorsal root ganglia (DRGs). When components of alpha-MEM are compared with those of other commercial media, nucleosides are found only in alpha-MEM. We examined the effects of nucleosides on myelin formation in rat DRG cultures. When myelin formation was viewed quantitatively, cultures using alpha-MEM with nucleosides yielded 2- to 3-fold more myelin segments than cultures using alpha-MEM without nucleosides.