Hepatopoietin A (HPTA) is an acidic heparin-binding polypeptide growth factor for hepatocytes with properties distinct from other known heparin-binding growth factors. HPTA is a heterodimer consisting of a heavy and a light polypeptide chain with Mr of 70,000 and 35,000 respectively. HPTA is a complete mitogen for hepatocytes in that it stimulates DNA synthesis in hepatocytes maintained in serum-free medium. Its complete purification from rabbit serum or human plasma was reported by us elsewhere (R. Zarnegar and G. Michalopoulos, 1989). In the present communication we report the N-terminal amino acid sequence of the HPTA light chain up to 24 residues (VVNGKPTRTNVGRMVSLKYRNKHI) and show that this sequence is unique and not related to any other proteins or growth factors based on computer search analysis. We have also raised antiserum against a synthetic peptide corresponding to the sequence of N-terminal amino acids residues 1 to 24, which recognizes the whole HPTA molecule. This antiserum as well as oligonucleotide probes corresponding to the N-terminal amino acid sequence of HPTA can be used as probes to identify tissue(s) of origin of this growth factor and assist in molecular cloning of its gene.