The proliferative activity of chick neuroblasts cultured in a medium containing a low (5%) or a high (20%) concentration of fetal calf serum (FCS) was analyzed and the influence of a chick brain extract was investigated. Morphological observations and tritiated thymidine incorporation measurements have shown that neuroblasts from 6 day-old chick embryo cerebral hemispheres proliferate more actively in the medium with 5% FCS compared to the medium with 20% FCS. The medium containing 5% FCS favoured the maintenance of neuronal cells in a neuroblast stage as shown by electron microscopy. The stimulatory effect of brain extract on the proliferation of neuroblasts is stronger in the low serum culture condition. These findings indicate that a low serum-containing medium is an adequate condition to study neuronal proliferation and effects of growth factors on these cells.