Antisera from various animal species containing antibodies to »Runde« virus were not able to neutralize virus infection in newborn mice, the outcome of which is an acute, fatal CNS disease. There was, however, one noticeable exception. Mixtures of virus and hyperimmune mouse serum or ascitic fluid inoculated intracerebrally into newborn mice resulted in a persistent infection and a chronic disease which had previously only been recognized in 2 to 3‐week‐old mice inoculated with »Runde« virus. A serum pool from persistently infected mice had the same effect, though this was less pronounced. The addition of unheated guinea pig serum to the virus‐hyperimmune serum mixtures reinforced the tendency to persistence and chronic disease, and unheated guinea pig serum alone modified the infection in the same way. The results suggest an immunological basis for the virus persistence and chronic disease in suck