Cultured rat Schwann cells are stimulated to divide by a protein growth factor, present in extracts of bovine brain and pituitary, which we have named glial growth factor (GGF). Two lines of evidence indicate that GGF activity in both brain and pituitary resides in a protein of Mr = 31,000. (1) Four independently isolated monoclonal antibodies that immunoprecipitate the activity react with an antigen of this molecular weight in sodium dodecyl sulfate (SDS)-polyacrylamide gels. (2) After SDS-polyacrylamide gel electrophoresis of partially purified preparations, mitogenic activity on Schwann cells is recovered at this molecular weight. GGF has been purified approximately 10(5)-fold to apparent homogeneity from bovine pituitary anterior lobes by a combination of column chromatography steps and preparative SDS gel electrophoresis. Purified human platelet-derived growth factor, a molecule with properties similar to those of GGF, is inactive on Schwann cells and therefore appears to be distinct.