A simple iodixanol density cushion can be used to concentrate the virus. Depending on the density of the cushion, such a technique may also partially purify the virus. A small volume of very dense cushion [1,2] was used for banding viral particles from a very large volume of virus-containing fluid. 

This stage can be carried out at 50,000 g for a short time (1.5-3 h). Should it be necessary to remove the iodixanol after recovery of the banded material, the virus can be efficiently pelleted from a small volume by gentle centrifugation for a long period [2]. Low-speed centrifugation of the original large volume on the other hand would lead to very poor recovery. 

These procedures lead to improved infectivity, compared to simple high-speed pelleting. Low-density cushions of 8.4% [3] and 6% [4] (w/v) iodixanol have been used to pellet HIV-1 and HIV-1 cores respectively and to separate them from more slowly-sedimenting contaminating particles and soluble proteins.