In this study, the proliferative activity of cultured chick neuroblasts exposed to high or low concentrations of fetal calf serum (FCS) was investigated. Different experimental evidences showed that the first 2-3 days of culture correspond to a period of neuronal proliferation in our culture conditions, whereas contamination by glial cells is very low. Cell counts and measurements of [3H]-thymidine incorporation have been used as an indicator of cell proliferation. With increasing concentrations of whole, dialyzed or desalted FCS the incorporation of radioactive thymidine and the number of cells were reduced. The inhibitory effect on [3H]-thymidine incorporation of whole serum was probably due to cumulative effects of the presence of free thymidine and of inhibitory components in that serum. An inhibitory factor from FCS has been partially purified by ammonium sulfate precipitation and column chromatographies on Ultrogel AcA 34, DEAE-cellulose and hydroxylapatite; its apparent molecular weight is about 90,000. The purified fraction was active at 2 micrograms/ml of culture medium.