Neutralizing antibody activity is present in serum and nasal secretions of cattle following IBR virulent virus challenge or intranasal vaccination with live, avirulent virus. Levels of nasal secretion antibody activity (NS-AA) are low and short-lived following initial exposure, but are considerably enhanced following reexposure provided infection occurs. NS-AA has not been detected in calves following parenteral administration of live virus vaccine. Such calves are subject to infection by challenge virus administered intranasally as early as three weeks after vaccination. Unlike parenterally administered vaccine, vaccine administeredintranasally promotes active immunity in young calves born of immune dams. Calves so vaccinated respond to later challenge with rapid, pronounced increases in both serum antibody and NS-AA without displaying overt signs of disease. Avirulent virus administered intranasally promotes release of interferon into nasal secretions for a period of six to eight days, coincident with continued virus replication. Classes of immunoglobulin involved in the various responses will be discussed.