Comparative investigations of the combined use of vaccines and interferon in tick-borne encephalitis, herpes, and acute encephalomyelitis of man showed that up to 78% of animal protection against 10--50 LD50 of intraperitoneally inoculated viruses could be achieved. Vaccination alone provided up to 56% survival of the infected mice, while administration of interferon subcutaneously or intraperitoneally in a dose of 800 units (53,300 units per 1 kilo body weight) 18--20 hours before virus gave a survival rate of 36%. The use of vaccine in combination with an interferon inducer, phage of f2 RNA, in tick-borne encephalitis gave up to 75% survival of the infected mice. Four hours after administration, interferon could be detected in mouse sera in a concentration up to 640 units/ml which in immune mice accumulated more rapidly and remained at a higher level 18 hours after inoculation of the inducer.