Density gradient ultracentrifugation has been widely used to fractionate animal, plant, and bacterial cells, viral particles, lysosomes, membranes, and mac-romolecules in a range of processes. Its application has been of particular significance in the commercial preparation of viruses for vaccine and immuno-therapy products in both batch and continuous-flow zonal modes. These methods have been traditionally used to purify influenza vaccines (1), but more recently have been used in devising new vaccine purifications such as Hepatitis B (2) and Rabies (3).