Proteopathic seeds are soluble species of aggregation prone molecules such as tau, that are competent to instruct endogenous proteins to undergo templated misfolding. Yet the idea of proteopathic seeds for tau is largely abstract, and the relationship of the seeds to the final fibrils that define the diseases has not been determined. We utilized mass spectrometry (MS), and in vitro bioassays to characterize the soluble tau species, derived from human Alzheimer brain, that are capable of inducing tau seeding in bioreporter cells and in vivo. Multiple post translational modifications (PTM) were identified by MS, including phosphorylations, acetylations, and ubiquitinations. While the patterns are quite similar to that of post translational modifications on insoluble paired helical filament preparations, the presence of fewer overall post translational modifications, and some unique modifications, distinguish soluble seed competent tau from filamentous tau in Alzheimer disease. Surprisingly, the presence of ubiquitin modifications on the soluble seed competent species correlates positively with bioactivity, and the stoichiometry of ubiquitin occupancy and other PTMs¬ correlate with the aggressiveness of clinical disease measured by age of onset and rapidity of progression. These results define the bioactive, seed competent tau species as being closely related to, but distinct from, mature paired helical filaments, and provide insight into the molecular features of tau PTMs that ultimately generate the bioactive conformation of tau present in Alzheimer disease (AD).