We have used the end-specific monoclonal antibodies to amyloid beta-protein (A beta), BC05 and BA27, to investigate the molecular characteristics of the cored or stellate type of amyloid plaque that is sometimes present, along with the more common diffuse type of plaque, in the cerebellar cortex in (usually younger) cases of Alzheimer's disease. In five such cases of Alzheimer's disease the (many) cored plaques were strongly BA27, but less strongly BC05, immunopositive, indicating the presence of (much) A beta 40 and A beta 42(43), respectively. Diffuse plaques were only BC05 positive, except on rare occasions where a litter BA27 reactivity was present. Cerebellar cored plaques, like the diffuse plaques, were not associated with tau or astrocytic (glial fibrillary acid protein) immunoreactivity, though in contrast to cerebellar diffuse plaques, but like the cored plaques in the cerebral cortex, microglial cells were usually present. The cause of this form of A beta deposition in the cerebellum is not known. Although congophilic angiopathy was severe in two patients, this was only mild in the other. Similar plaques were also seen in the cerebellum of most, but not all, of five other younger patients with chromosome 14-linked Alzheimer's disease and again, although congophilic angiopathy was severe in one such case with many cored plaques, this was not so in the others. At present the relationship (if any) between this pathological change and the possession of the chromosome 14 mutation of Alzheimer's disease or the occurrence of congophilic angiopathy remains uncertain.