Histological examination of lungs collected from mice inoculated with influenza A/PR 8/34 (H0N0) and/or parainfluenza type 3 viruses revealed the existence of lymphohistiocytic interstitial bronchopneumonia lesions. Necrosis zones were found in the lungs of influenza virus infected animals. Associated infection by the two viruses induced also the fibroblast and fibrocyte proliferation, at same average ratios.