The neuronal proteins Tau and MAP2 use homologous C-terminal MT-binding regions (MTBRs) to interact with microtubules, F-actin, and intermediate filaments. Although Tau-MTBR is the principal component of pronase-treated Alzheimer paired helical filaments, both Tau and MAP2 form filaments in vitro from disulfide-linked homodimers. That the critical thiol lies within a domain needed for MT binding raised the question: Does disulfide formation block Tau-Tau or MAP2-MAP2 dimer binding to microtubules, thereby acting to divert dimers toward filament formation? We now report that cross-linked Tau and MAP2 homodimers readily promote tubulin polymerization and that monomer and dimer affinity for MTs is surprisingly similar. Therefore, disulfide cross-bridging into homodimers is unlikely to be a drive force for filament formation in Alzheimer's disease.