Diffuse interstitial pneumonia represents a common clinical, radiologic and pathological entity. A great proportion of cases are idiopathic. Hereditary predisposition, auto-immunity, and viral infection have been considered as possible etiologic agents but there has been no specific demonstration or exclusion of their role of these factors in the idiopathic interstitial lung diseases. Alveolar wall immune complex deposition will be determined utilizing the direct technique for immunofluorescence. Fluoresceinated antisera to the major immunoglobulins and complement will be exposed to biopsied lung of subjects with interstitial pneumonia. In addition, immune complex deposition in the lung will be corrected with the presence of circulating immune complexes (Raji cell technique). Immune electronmicroscopy will also be performed on the lung biopsies. The role of heredity will be assessed by histocompatibility typing. Viral cultures will be performed on all lung biopsies as well as exposing the lung to specific antiviral fluoresceinated antisera.