Antiserum to synthetic pentadecapeptide corresponding to the 423-437 sequence of putative human c-myc gene product stained cells in a human hepatoblastoma transplanted in athymic nude mice. The immunostained cells were found in clus-ters and mainly distributed in areas adjacent to the blood vessels and lakes. The immunoreactive elements were localized in the cytoplasm of the tumor cells. Cellular myc (c-myc) gene is a homolog of avian MC29 viral transforming gene (v-myc). The nucleotide sequence of human c-myc oncogene has been determined by the gene cloning technique (2, l2) and the protein coded by the gene was deduced to consist of 439 amino acidlresidues. On the basis of the structural information of human c-myc gene product, we synthesized a pentadecapeptide corresponding to the C-terminal 423-437 sequence of the predicted protein and produced an antiserum against the synthetic peptide. With use of the antiserum the presence of c-myc-related protein was demonstrated immunohistochemically in a human hepatoblastoma