Immunoblots from embryonic chicken optic lobes (midbrain) and spinal cord were found to contain different proteins with a molecular weight between 45 and 70 kDa that are recognized by monoclonal antibodies (mAbs) directed at human tau proteins. Each appears as a doublet on SDS-PAGE. The upper, slower migrating bands were recognized by AT8, a monoclonal antibody that requires a phosphorylated Ser-202, an epitope specific for abnormally phosphorylated tau of paired helical filaments (PHF-tau) in Alzheimer brains. The corresponding faster migrating bands were stained by the mAbs BT2 and tau-1, both requiring an epitope containing a non-phosphorylated Ser-202. Phosphatase treatment abolished binding of AT8 and induced an additional binding of tau-1 (and BT2) to the upper bands. Thus, the data suggest that in embryonic chicken central nervous system Ser-202 occurs in a phosphorylated and in a non-phosphorylated state in several distinct tau isoforms.