The continued administration of concentrated interferon preparations prolonged the mean survival time of BALB/c mice infected with Rauscher virus. The evolution of various manifestations of Rauscher disease—hepatosplenomegaly, leulcocytosis, and anemia—was also delayed in interferon-treated mice as was the development of sequential pathologic changes in the spleen. Less infectious virus was present in the spleens of several interferon-treated mice than in the spleens of controls, as determined by biologic assay and electron microscopy. It is suggested that continued interferon treatment delays the progression of Rauscher disease by a continued inhibition of the multiplication of Rauscher virus.