Two types of tau isoform, three- and four-repeat tau, are found in neurofibrillary tangles-a pathological hallmark of tauopathics. Which isoform is deposited in the affected tissues depends on the tauopathy. To study how and which tau isoforms contribute to neuronal degeneration, we have developed and characterized two novel conformation-sensitive antibodies, T3R and T4R. Two closely related synthetic peptides, PGGGKVQIVYK and PGGGSVQIVYK, respectively, were designed as antigens. The isoform-specific residues, K-305 in three-repeat tau or S-305 in four-repeat tau, and the PHF6 motif (VQIVYK) were identified as critical sequences. Despite the high similarity of the antigens, there was no cross-reactivity between T3R and T4R. Furthermore, T3R and T4R showed reduced binding to the thioffavin-positive beta-structural form of their target. These features may enable these antibodies to act as novel indicators that allow us to observe and evaluate conformational changes in each distinct isoform of tau. (C) 2007 Elsevier Inc. All rights reserved.