Extract was prepared from bovine brain by treatment with chloroform-methanol, drying of the precipitate, and successive extractions of the powder with Tris buffer. Influence of this extract on neuroblast proliferation in culture was tested. Neuroblast cultures were prepared from brain hemispheres of 6-day-old chick embryos. Cell proliferation was quantified by incorporation of 3H-thymidine. The brain extract was able to stimulate the proliferation of the neuroblasts in culture. In order to purify the factor responsible of this effect, the brain extract was subjected to diethylaminoethyl (DEAE)-cellulose chromatography and gel filtrations. By UV spectrography and destruction of the activity with RNAase and KOH, it was shown that in the extract RNAs are responsible for at least a part of the effect on the proliferation of neuroblasts in culture. Brain RNA is not specific for this effect, since mouse ribosomal RNA was also active. The effect could be actually elicited by some nucleotides.