A purification procedure and partial characterization of bovine pituitary fibroblast growth factor (FGF) are described. The steps of the published methods [3,4] which yield inhomogeneous material, were retained, with modifications. The final isolation, with an additional purification of approximately 20-fold, was achieved by electrophoresis in polyacrylamide gels at acid pH. The mitogenic peptide has a molecular weight of 14,500--15,000 as determined on SDS gels, chromatographs as a monomer in nondenaturing conditions, and is active at the picomolar level in effecting the incorporation of 3H-thymidine in Balb/c 3T3 cells. A preliminary amino acid composition is presented.