At least 22 different mutations associated with early-onset familial Alzheimer's disease (AD) in various kindreds have been reported to occur in a recently identified gene on chromosome 14, presenilin 1 (PS-1) (Sherrington et al. (1995) Nature 375, 754-760  and reviewed by Van Broeckhoven (1995) Nat. Genet. 11, 230-231 ). In order to study the localization of PS-1 in the brain, we raised a polyclonal antiserum specific to a fragment of the predicted protein sequence of PS-1. PS-1 immunostaining was found intracellularly, in the perikaria of discrete cells, mostly neurons, appearing as thick granules, resembling large-size vesicles. These granules were located in the periphery of cell bodies and extended into dendrites and neurites. PS-1 expression was found to be broadly distributed throughout the mouse brain, not only in structures involved in AD pathology, but also in structures unaltered by this disease.