A viral inhibitor displaying the main properties of a type 1 (alpha or beta) interferon was found in intestine and serum of diarrheic newborn calves following oral administration of bovine rotavirus. Other internal organs examined 24 hours after infection all contained appreciable amounts of interferon. A kinetic study performed in ileum and serum of a one day-old calf showed a good time correspondence between peak levels of virus multiplication in the gut, intestinal interferon, and systemic interferon. In the same experiment, a two week-old calf was refractory to the disease, developing much delayed and weaker productions of both virus and interferon, when compared to the newborn calf. Therefore, the enhanced susceptibility of the newborn calf to rotavirus induced diarrhea is not related to a deficiency in interferon synthesis.