SummaryType I interferons (IFNs) inhibit viral replication and cell growth and enhance the immune response, and therefore have many clinical applications. IFN-α 2b ranks third in world market use for a biopharmaceutical, behind only insulin and erythropoietin. The average annual cost of IFN-α 2b for the treatment of hepatitis C infection is $26 000, and is therefore unavailable to the majority of patients in developing countries. Therefore, we expressed IFN-α 2b in tobacco chloroplasts, and transgenic lines were grown in the field after obtaining