Neuronal inclusions with bundles of abnormal filaments made of tau polymers are found in numerous diseases with neurofibrillary degeneration. Tau proteins are the basic components of paired helical filaments (PHF) in Alzheimer's disease (AD), and are abnormally phosphorylated. A disease-specific phosphorylation site at serine422 was demonstrated on PHF, but not on tau proteins from biopsy-derived brain samples. In the present study, we report the characterization of a polyclonal antibody (988) against the serine422 phosphorylation site. By using biochemical and immunohistochemical methods, we confirmed that it is not found on tau proteins from biopsy- or autopsy-derived control samples, and we investigated the presence of this epitope on tau proteins in several neurodegenerative disorders, including AD, Down syndrome (DS), Guamanian amyotrophic lateral sclerosis/Parkinsonism-dementia complex (ALS/PDC), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), postencephalitic parkinsonism (PEP) and Pick's disease (PiD). By Western blotting, antibody 988 labeled the characteristic tau triplet (tau 55, 64, 69) in AD, DS, Guamanian ALS/PDC and PEP. PSP and CBD exhibited their typical tau doublet (tau 64, 69), whereas the doublet tau 55 and 64 was detected in PiD. In all of these neurodegenerative disorders, antibody 988 clearly labeled NFT and dystrophic neurites, as well as Pick bodies in PiD cases, whereas no staining was observed in control cases. These data indicate that phosphorylation of serine422 on tau proteins is a common feature among neurodegenerative disorders and is therefore not specific of AD. Moreover, phosphorylation of this epitope permits the distinction between normal tau proteins and pathological tau proteins.