Growth inhibitory factor for mouse neuroblastoma cells was detected in the culture medium of fetal rat glioblasts, and was partially purified and characterized. The factor had an apparent molecular weight of 75,000 and an isoelectric point of 5.8, and showed no esterase activity. It possessed the activity to promote morphological differentiation including the formation of neural processes and the inhibition of cell division when tested on mouse neuroblastoma cell lines (Neuro 2a, NS-20Y, and NIE-115). The activity was susceptible to protease digestion and heat treatment. The serum over 25% cancelled the inhibitory activity of this factor. The factor failed to increase the intracellular contents of cAMP and cGMP. It showed no effect on either morphological differentiation or proliferation of glial cell lines, suggesting that under physiological conditions the factor acts specifically on neuronal cells.