The degree to which histopathological lesions correlated with the numbers ofM. ovipneumoniaeand bacteria was studied in sixty 6 to 9 month-old lambs with chronic and subacute pneumonia slaughtered at a local meatworks. Large numbers ofM. ovipneumoniaewere associated with chronic proliferative changes such as peribronchiolar fibrosis and alveolar interstitial thickening. The combined effect of large numbers ofM. ovipneumoniaeand bacteria were associated with neutrophilic exudation and epithelial hyperplasia. However, lymphoid hyperplasia and excess mucus production were associated with low bacterial titres. There was no direct correlation between the numbers ofM. ovipneumoniaeand bacteria present in the pneumonic lungs.