Abstract: The invention provides new methods for purifying and concentrating viruses. The inventors have discovered that high molecular weight proteoglycans present in retroviral stocks are co-concentrated with the retroviruses, and can inhibit retroviral transduction. The new purification and concentration methods feature treatment of virus stock with an anionic polyelectrolyte and a cationic polyelectrolyte, followed by centrifugation. The new methods minimize the amount of proteoglycan co-precipitated with the infectious virus.