In Alzheimer's disease (AD) neurofibrillary tangles (NFT) are formed by hyperphosphorylated microtubule-as sociated tau protein. It is still a matter of controversy which phosphorylation sites are AD-specific and how these might be linked to the cause or progress of the disease. Whereas most research projects in this field rely on phosphorylation-dependent tau-specific monoclonal antibodies (mAbs), the phosphorylation patterns recognized by these mAbs are often not characterized in detail. Therefore, we synthesized unphosphorylated, two monophosphorylated (pThr231, pSer235), and the bisphosphorylated (pThr231 + pSer235) tau226-240 peptides. The phosphopeptides were ligated via an N-terminal cysteine to the thioester-activated C-terminus of human aldo/keto reductase AKR1A1. After purification by preparative gel electrophoresis, the ligation products were analyzed by Western blotting and probed with phosphorylation-dependent anti-tau mAbs HPT-101, HPT-103, HPT-104, and HPT-110. The obtained specificities were very similar to the data obtained by ELISA, showing that ELISA-based epitope mapping studies are also valid for immunoblot analyses. (c) 2007 Elsevier Inc. All rights reserved.