Previous infection studies have invariably assigned a minor role to interferon- λ (IFN- λ ) in the control of influenza viruses in the lungs of mice. However, when we administered the virus selectively to the upper respiratory tract, we came to a completely different conclusion. Under such more physiological conditions, influenza virus replication was largely restricted to the nasal tissue in wild-type mice. In contrast, in mice lacking functional receptors for IFN- λ , influenza virus was frequently observed to spread to the lungs. Interestingly, the virus spread more frequently to the lungs in mice lacking functional receptors for IFN- λ than in mice lacking functional receptors for IFN- α / β , indicating that IFN- λ is more critical for virus restriction in the upper respiratory tract. We further observed that the Udorn (H3N2) influenza A virus strain is readily transmitted upon contact among mice lacking functional receptors for both IFN- α / β and IFN- λ . Contact transmission of this virus was also efficient when mice lacking functional receptors for IFN- λ were employed. In contrast, wild-type mice and mice lacking functional receptors for IFN- α / β transmitted the virus only with reduced efficacy. These findings demonstrate that the crucial role of IFN- λ in influenza virus restriction is only getting apparent under experimental conditions which imitate the physiological route of virus infection.