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Summary 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. In a previous publication (Baskerville, Baskerville & Wood, 1982) we described histopathological and bacteriological studies of naturally-occurring chronic pneumonia in a colony of guineapigs. The lesions were of two main types, pulmonary interstitial fibrosis (PIF) and perivascular lymphoid hyperplasia (PVL). No correlation was found between the presence of Bordetella bronchiseptica or other bacteria and the development of the pulmonary lesions, and their aetiology remains unknown. A detailed electron microscopical study was therefore undertaken to determine the nature and pathogenesis of these chronic lesions and to search for evidence of other possible causative agents, in particular mycoplasmas and viruses.