It was possible to grade the degree of pulmonary infection into five groups(Blyth, W. 1972). Two examples are seen in sections stained with haematoxylin and eosin. Individual inflammatory cells and basic alveolar structure are shown by transmission electron microscopy. Four days after challenge in Grade 5 infections there is widespread infitration of alveolar septa by neutrophil polymorphonuclear leucocytes accompanied by a few eosinophils. Perivasculitis and peribronchiolitis are common during widespread alveolar filling and haemorrhage . By fifteen days, polymorphonuclear leucocytes are rare. Macrophages are predominant in cell infiltrates which include lymphocytes and plasma cells. In contrast, Grade3 infections cause only sporadic areas of perivascular and peribronchiolar involvement . Areas of alveolitis are diffuse with occasional areas of alveolar filling.