Optimal bioassay conditions for bovine glia maturation factor (GMF) were determined among glial cells from normal glioblasts to glioma cells. Rat glioblasts 4–8 days after subculture show the highest response to GMF with regard to morphological transformation and mitogenic activity. Bovine GMF enhances DNA synthesis of rat glioblasts at 12 hr after stimulation; maximum incorporation of [methyl-3H]thymidine was detected at 18 hr. GMF increases twofold the saturation density of rat glioblasts but does not alter that of C6 astrocytoma cells. The apparent inhibition of mitogenic activity of high doses of GMF is seen in both normal and malignant glial cells.