Neuronal Precursor Cell Differentiation Supplement (NPCDS) is a medium supplement designed for the optimal differentiation of normal human neuronal precursors in vitro. It is a sterile, concentrated (100X) solution which contains growth factors, hormones, and proteins necessary for the culture of normal human neurons. The supplement is formulated (quantitatively and qualitatively) to provide a defined and optimally balanced growth environment that maximally promotes the differentiation of normal human neurons in vitro. The supplement is designed as an additive for neuronal precursor cell differentiation medium (NPCDM, Cat. No. 1531) and should be used in conjunction with that medium.