Summary:A novel thiazolopyrimidine nucleoside, 5‐amino‐3‐&bgr;‐d‐ribo‐furanosylthiazolo [4, 5‐d]pyrimidine‐2, 7 (3H, 6H)‐dione (7‐thia‐8‐oxoguanosine), was evaluated for antiviral activity in rodent models, and at 50‐200 mg/kg prevented death in mice inoculated intraperitoneally (i.p.) with Semliki Forest, San Angelo, and banzi viruses when administered i.p. before virus challenge. Similarly, the nucleoside was effective against an intranasal challenge of rat coronavirus in suckling rats, with activity present when treatment started as late as 4 h after virus inoculation. Protection was observed against herpes type 1 and murine cytomegalovirus (both inoculated i.p.) infections, and encephalitis induced by intracerebral inoculation of a human coronavirus in mice. Friend leukemia virus splenomegaly was more severe in drug‐treated animals than in placebos. This immune modulator is promising for the treatment of animal and human diseases.