Complement (C) titers were decreased at 3 days post-infection, and virus-neutralizing (VN) antibody was detectable at 3 or 4 days post-infection in chickens with infectious bursal disease virus (IBDV). Clotting times were prolonged in all groups tested during the acute phase of the disease. Mortality appeared associated with the severity of decrease in C titer. The mortality and many of the clinical signs seen with infectious bursal disease may be associated with a depletion in circulating levels of hemolytic C from the formation of immune complexes at sites of viral replication and/or a depletion in some clotting factor which results in hemorrhage.