The recent identification of the Huntington's disease (HD) gene, enabled us to synthesize oligopeptides corresponding with the carboxy-terminal end of the predicted HD-gene (IT15) product. Immunobiochemical studies with polyclonal antibodies directed against this synthetic peptide (position 3114–3141) on lymphobiastold cells from normal individuals and patients with Huntington disease, revealed the presence of a protein (huntingtin) with a molecular mass of approximately 330 kDa. Immunocytochemical studies showed a cytoplasmic localization of huntingtin in various cell types including neurons. In most of the neuronal cells the protein was also present in the nucleus. No difference in molecular mass or intracellular localization was found between normal and mutant cells.