An infectious disease of calves has been described which is characterized by fever, diarrhea, and pneumonia, followed soon by recovery. On autopsy of animals killed at the height of the disease, there is found a catarrhal enteritis and a bronchopneumonia that is usually confined to the anterior lobes of the lungs. From this disease an agent has been secured by the serial inoculation of lung extracts that produces a pneumonia in white mice. Attempts to demonstrate by the same means a similar agent in uninoculated mice from the same stock have yielded negative results. Suspensions of the lungs of the mice with pneumonia, when inoculated intranasally or intratracheally into calves, cause a disease like the natural infection, characterized by fever, diarrhea, and pneumonia. In two experiments pen contact of normal calves with calves inoculated with the passed material resulted in the typical disease. Early in its course the causative agent is found only in the lungs and intestines, but at its height is generally distributed throughout the body. Calves that have recovered from the induced disease are resistant to subsequent infection and their blood serum will neutralize the causative agent as not previously. Sera from calves that have recovered from the natural disease also neutralize the agent. Cultures from the infected lungs of calves and mice as a rule show no growth, and material that has been passed through Berkefeld N filters produces the characteristic disease. It is therefore concluded that this disease of calves is caused by a filterable virus.