Abstract: Cloned, i.e. defined, defective interfering (DI) influenza A virus is produced in embryonated hens eggs using a method which generates large quantities of DI virus material. Co-administration of cloned DI influenza A virus with a lethal dose of virulent influenza A virus conferred protection in mice compared to a control of inactivated cloned DI influenza A virus. Control mice which received only cloned DI influenza A virus and no lethal challenge of virulent influenza A virus were not protected three weeks later on lethal challenge with infective virus. Cloned DI influenza A virus of one subtype is found to act in vivo as an effective antiviral against the same or any other sub-type of influenza A virus. The antiviral effect has been found to have both a therapeutic and a prophylactic application against influenza A infection in humans, mammals and birds.