The distribution of abnormally phosphorylated tau proteins was investigated in a number of cortical areas and in the basal nucleus of Meynert of 12 patients with Alzheimer's disease. Quantification was performed with a sensitive sandwich enzyme-linked immunosorbent assay employing the monoclonal antibody B5-2, which immunoreacts with neurofibrillary tangles, dystrophic neurites surrounding amyloid cores of plaques and neuropil threads. On western blots, the monoclonal antibody B5-2 specifically labels the abnormally phosphorylated paired helical filament-tau species in a phosphorylation-dependent manner but is not cross-reactive to normal tau proteins. The preferential involvement of cytoarchitecturally defined cortical areas showed marked individual differences in Alzheimer's disease, and no unique distribution pattern of abnormally phosphorylated tau proteins could be established. While regional heterogeneities along the rostrocaudal extension of the brain declined with the progression of the disease, lateral differences which were largely non-directional appeared to be more stable over time. The mean content of abnormally phosphorylated tau proteins of individual cases was significantly related to the severity of the disease. On a regional scale, this correlation was highest for the basal nucleus. The formation of abnormally phosphorylated tau was associated with a loss of normal soluble tau proteins. Those cortical areas which in normal brain showed the highest amount of normal soluble tau proteins appeared to be particularly prone to deposition of abnormally phosphorylated tau proteins. The present results indicate that the formation of abnormally phosphorylated tau proteins can be initiated in the neuropil at more than one brain area. The spreading of the pathology appears to involve both intracortical and subcortical pathways but largely spares interhemispheric pathways. It is hypothesized that regional differences in the compartmentation and metabolism of tau proteins might reflect the molecular basis for the regionally different vulnerability in Alzheimer's disease.