Abstract Neuroblasts from cerebral hemispheres of 6-day-old chick embryo are able to divide to a certain extent under suitable culture conditions. It was found that addition of purine bases to the culture medium induced an increase of tritiated thymidine incorporation into the cells, resulting from a stimulation of neuroblast proliferation. Most purines elicited a stimulation, but guanine compounds were the most active. Inosinic acid (IMP), the first purine synthesized, was also active. Folic acid was inactive. These results suggest that neuroblasts in culture are defective in the biosynthesis of purines and that this deficiency is not due to a lack of folic acid. Some other cell types were also tested including glial cells, meningeal cells, whole embryo fibroblasts and heart fibroblasts. Only the latter did not respond to purine bases. These observations show that different cell types in primary culture need exogenous purines for maximal growth.