The presence of a high molecular weight polypeptide growth factor in the plasma of normal human or rat serum which stimulates DNA synthesis in primary cultures of normal rat hepatocytes has previously been reported. This activity was referred to as Hepatopoietin A (HPTA). At that time, however, complete purification of this growth factor had not been achieved. In the present report the steps required for complete purification of HPTA from human plasma or rabbit serum are described. The purification involved sequential ammonium sulfate precipitation, heparin-affinity chromatography, anion exchange HPLC, and reverse-phase HPLC. The final purified product is a heterodimer consisting of a heavy and a light polypeptide chain with M.W. of 70,000 and 35,000 respectively as determined by SDS-PAGE under reducing conditions. Under non-reducing conditions, however, the purified HPTA migrated as a single band on SDS-PAGE corresponding to a M.W. of 69,000. The mitogenic activity of HPTA was associated with this band when it was eluted from unstained SDS-polyacrylamide gels. Gel filtration HPLC under neutral isotonic conditions indicated that HPTA tends to form aggregates with M.W. of greater than 300,000. Chromatofocusing indicated that HPTA is an acidic protein with a PI of about 5.5.