Abstract
Pulmonary interstitial fibrosis commenced as focal proliferation of fibroblasts and accumulation of macrophages in interalveolar septa and was followed by deposition of excess collagen. Epithelium of airways and alveoli remained normal. Perivascular nodules of lymphoid tissue which developed in the lungs of many older animals had a high mitotic rate, but the lymphocytes were morphologically normal. Micro-organisms were not observed in the lungs of any animal.