The invention provides a process for the production of polio virus, comprising the steps of: a) providing a serum-free suspension culture of cells, which are PER.C6 cells, b) infecting said cells with polio virus, at a cell density of between 2x106 cells/ml and 15Ox106 cells/ml, and c) harvesting polio virus at a time of between 12 and 48 hours after infection.