In mice given single intraperitoneal doses of interferon, serum interferon levels peaked at 1 h postinjection and were reduced to zero at about 8 h. The interferon concentrations in spleen, liver, and lungs were about 100-fold higher than could be expected from the amount of serum contained in these organs. In the brain only low levels of antiviral activity were detected. In mice infected intraperitoneally with Mengo virus, viral replication in the brain occurred around day 4 and was accompanied by the appearance of large amounts of interferon (approximately 10(3.25) U/g). This was preceded, however, by viral replication in the spleen and by the appearance of modest amounts of interferon in spleen and serum. In these mice protection could be obtained with relatively small doses of interferon, provided protection could be obtained with relatively small doses of interferon, provided they were given before the time of maximal levels of endogenous serum interferon. In mice infected intranasally with vesicular stomatitis virus, virus replication in the brain started within 24 to 48 h and increased with time; also, small amounts of interferon (10(2) to 10(2.5) U/g) were already detectable on days 1 and 2. The major peak of virus replication in the brain occurred on days 5 to 6 and was accompanied by the appearance of large amounts of interferon (approximately 10(3.25) U/g). In this model early treatment with interferon also provided protection, but only if given in larger doses than in the Mengo virus system. Athymic (nu/nu) mice developed a chronic systemic infection when inoculated with a demotropic strain of vaccinia virus. No interferon was detected in sera, livers, spleens, or lungs of these animals; some mice had low levels of interferon-like antiviral activity in the brain, but no attempt was made to characterize this material. Daily administration of large doses of interferon failed to exert an effect on the development of this chronic disease. Yet, normal (NMRI) mice were protected against acute infection with dermotropic or neurotropic strains of vaccinia virus, and athymic mice were partially protected against acute lethal infection with neurotropic vaccinia virus.