Guinea pigs, rhesus monkeys and marmosets were infected with L. pneumophila in small particle aerosols. Fever and acute fibrinopurulent pneumonia resulted. The lesions involved distal lung tissue only, spreading from terminal and respiratory bronchioles and producing heavy infiltration of alveoli by polymorphs and macrophages and widespread exudation of oedema fluid and fibrin. Lesions were not found in extra-pulmonary sites.