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