The invention provides a process for the production of polio virus, comprisingthe steps of: a) providing a serum--freesuspension culture of cells, which are PER.C6 cells, b) infecting said cellswith polio virus, at a cell density of between 2x10 6cells/ml and 15Ox10 6 cells/ml, and c) harvesting polio virus at a time ofbetween 12 and 48 hours after infection.