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Systemic toxicity currently prevents full clinical application of type I interferons (IFNs) for the treatment of cancers and viral infections. Although the use of IFNs fused to antibodies resulted in a 10-fold increased activity towards targeted cells, off-target side effects experienced by the patients are expected to remain a major obstacle. We present a novel strategy to engineer IFN α with high in vitro and in vivo activity towards cells expressing a specific surface marker while having an up to 1000-fold reduced activity towards non-relevant cells. The concept lies in the use of immunocytokines based on IFN mutants with strongly reduced binding affinity for the IFN receptor. Such a targeted IFN recovers almost maximally its antiviral activity uniquely on the targeted cells and acts to selected cell populations in mice with targeting efficiencies up to 3 orders of magnitude. This novel concept holds promise to revitalize the clinical potential of IFN α and other cytokines.