Transcellular propagation of protein aggregates—or seeds—is increasingly implicated as a mechanism for disease progression in many neurodegenerative disorders, including Alzheimer’s disease and the related tauopathies. While neuropathology generally originates in one discrete brain region, pathology progresses as disease severity advances, often along discrete neural networks. The stereotypical spread of tau pathology suggests that cell-to-cell transfer of toxic protein aggregates could underlie disease progression, and recent studies implicate seeding as a proximal marker of disease, as compared to standard histological and biochemical analyses. Commonly used metrics for protein aggregation detection, however, lack sensitivity, are not quantitative, and/or undergo subjective classification. Here, we describe a FRET and flow cytometry cell-based assay that allows for rapid and quantitative detection of protein aggregates from human and rodent biological specimens.