We have isolated a basic polypeptide from normal human serum that induces contact-inhibited cells to synthesize DNA and divide; this polypeptide is not necessary for the growth of viral-transformats. It has been obtained from pooled normal human sera by ion exchange chromatography (Dowex-50w-x8, Na ion cycle) and has been further purified by gel filtration and preparative isoelectric focusing on polyacrylamide gel. The molecular weight of the basic polypeptide is about 13,000 daltons as judged by polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS). Isoelectric focusing studies have shown that the active polypeptide has a pI of 9.7. Analytical isoelectric focusing studies and SDS electrophoresis indicate that this polypeptide consists of a single component. In addition, our studies have shown that this basic polypeptide is free of insulin-like activity and that it is a distinct entity separate from other polypeptides with insulin-like activity. It is stable to heating at 98 degrees C for 20 minutes, but its activity is destroyed by treatment with mercaptoethanol and by incubation with chymotrypsin and trypsin.