A number of different dose schedules of a potent interferon stimulator, polyinosinicδpolycytidylic acid, were compared in Semliki Forest virus infected mice. Daily treatment with 100 μg of InδCn intraperitoneally was more effective than any of the other dose schedules tested. Using this regimen it was possible to significantly decrease the mortality of SFV infected mice when treatment was begun as late as 5 days after virus inoculation, at a time when high titers of virus were present in most brains. In experiments where the interferon stimulator gave rise to more interferon than had been applied passively in previous studies, the inducer exerted a much greater protective effect.