Summary Ehrlich ascites tumors grew equally well in ICR and inbred A2G mice. ICR mice were sensitive, A2G mice were fully resistant to WSA influenza virus given intracerebrally. When mice bearing Ehrlich ascites tumors were inoculated intraperitoneally with WSA virus, rapid oncolysis ensued. In ICR mice this invariably led to death of the animals, while around 80% of A2G mice survived. Of these, some later succumbed to slowly growing subcutaneous tumors developing at the site of primary tumor implantation. This occurred frequently when tumors were either treated early with large doses of virus, or late with small doses. With other combinations of timing and dosage, some mice developed subcutaneous tumors which later regressed. Mice surviving the primary tumor inoculation for 1 to 4 months were resistant to challenge with the same tumor.