Alzheimer’s disease (AD) is, in part, defined by the polymerization of tau into paired helical and straight filaments (PHF/SFs) which together comprise the fibrillar pathology in degenerating brain regions. Much of the tau in these filaments is modified by phosphorylation. Additionally, a subset also appears to be proteolytically truncated, resulting in the removal of its C terminus. Antibodies that recognize tau phosphorylated at S^396/404 or truncated at E³⁹¹ do not stain control brains but do stain brain sections very early in the disease process. We modeled these phosphorylation and truncation events by creating pseudo-phosphorylation and deletion mutants derived from a full-length recombinant human tau protein isoform (ht40) that contains N-terminal exons 2 and 3 and all four microtubule-binding repeats. In vitro assembly experiments demonstrate that both modifications greatly enhance the rates of tau filament formation and that truncation increases the mass of polymer formed, as well. Removal of as few as 12 or as many as 121 amino acids from the C terminus of tau greatly increases the rate and extent of tau polymerization. However, deletion of an additional 7 amino acids, ³¹⁴DLSKVTS³²⁰, from the third microtubule-binding repeat results in the loss of tau’s ability to form filaments in vitro. These results suggest that only part of the microtubule-binding domain (repeats 1, 2 and a small portion of 3) is crucial for tau polymerization. Moreover, the C terminus of tau clearly inhibits the assembly process; this inhibition can be partially reversed by site-specific phosphorylation and completely removed by truncation events at various sites from S³²⁰ to the end of the molecule.