The high-resolution size capabilities of the Zetasizer Ultra have enabled a new assay to rapidly characterize Adeno Associated Virus (AAV) viral concentration. Utilizing Multi Angle Dynamic Light Scattering (MADLS), the Zetasizer Ultra can determine both AAV viral concentration and size in a single measurement that is cuvette-based, rapid, label-free, low volume, and non-destructive. In this talk, we present AAV viral concentration and size results, the MADLS measurement principle, and the effect of sample properties, such as material refractive index and viscosity.