A monoclonal antibody (mAb), HPC-1, labels the plasma membrane of the amacrine cell soma and inner plexiform layer in rat retina and other central neurons. HPC-1 antigen recognizes several proteins of about 35 kDa. In this study, an HPC-1 positive cDNA, HPC-113, was isolated from a lambda gt11 cDNA library of the rat hippocampus. HPC-113 had the 894-base pair nucleotide sequence in an open reading frame and the calculated molecular mass of the deduced amino acid sequence (298 residues) was 33,989 Da, implying that HPC-113 contains almost the full-length coding region of HPC-1 antigen is an integrated membrane protein revealing the characteristic alpha-helical structure with periodical heptad repeats usually seen in proteins with coiled-coil structures. Although the entire amino acid sequence did not show significant homology to any proteins so far known, a few local sequences in the possible extracellular domain of the HPC-1 antigen molecule had notable homology to some partial sequences in the laminin B1 chain. These sequences of laminin are included in the portion which has neurite outgrowth and/or survival promoting activity. The HPC-1 gene was transcribed in nerve tissues much more predominantly than in non-neuronal tissues. Thus, HPC-1 antigen(s) was confined to be a newly identified neuronal cell membrane protein(s) localized in a subpopulation of neurons.