Four monoclonal antibodies (1D2/1/2, 1G10/2/3, 3B6/1/1, 4D12/2/6) were raised against a synthetic peptide consisting of residues 8-17 of a protein reported to be common to senile plaque cores, cerebrovascular amyloid and neurofibrillary tangles in Alzheimer's disease. In an immunoperoxidase study of Alzheimer brain tissue, these antibodies stained plaque and vascular amyloid but not tangles, suggesting that the polypeptide chain in the region of residues 8-17 is exposed in the former two but, if present, inaccessible in the latter. In addition, staining of granular material in the plaque periphery was observed. These antibodies will be useful tools for future work on the origin of this protein.