Abstract. Out of 45 cases of fatal chronic pneumonia in calves examined for Mycoplasma bovis infection from February to July 1994, 11 cases with pulmonary abscesses that were culture positive for M. bovis were encountered. The cases were studied in detail using a recently developed monoclonal antibody-based immunoperoxidase technique. Mycoplasma bovis organisms were detected in specific locations at all stages of abscessation observed. In bronchioles or terminal airways within which abscesses developed, M. bovis was located at the epithelial surface and in close association with infiltrating neutrophils and macrophages. Abscessed airways that had lost the epithelium were encapsulated and were seen as coagulative necrotic foci that stained intensely for M. bovis, especially at the periphery. Some foci stained weakly and such might have been resolving lesions. Mycoplasma bovis was also demonstrated at sites of mild mononuclear cell infiltration in the livers and kidneys of 2 calves. The mycoplasma was detected within bile ducts in the liver and in the tubular epithelium of the kidney. Abscesses not staining for M. bovis, presumably caused by other pathogens, were seen concurrently with M. bovis-associated abscesses in some lungs. Thirteen other M. bovis-positive cases showed no abscesses, possibly indicating heterogeneity among M. bovis strains. Three other cases with abscesses were negative for M. bovis by culture and immunoperoxidase staining. The monoclonal antibody-based immunohistochemical technique is efficient for specific detection of M. bovis in cases of enzootic pneumonia of calves with or without abscessation. Mycoplasma bovis is implicated in the pathogenesis of lung abscesses in some calves.Naturally occurring infectious pneumonias of clinical significance usually have complex causes, often requiring the interaction of two or more organisms and predisposing environmental factors.2 Calf pneumonia resistant to antibiotic therapy is frequently encountered in the United States of America and Canada. 10 In areas where calves dying of chronic pneumonia would normally have been treated for bacterial agents, diagnostic effort is necessarily geared towards elucidating the role of viruses and mycoplasmas. Viral agents often cause acute pneumonia in calves, with a histological picture of bronchointerstitial pneumonia. 2Among bovine mycoplasmas recognized in the USA, Canada, and Europe, Mycoplasma bovis stands out as the most invasive and destructive. This mycoplasma can cause pneumonia and arthritis in calves and mastitis in adult cattle .2,7 Mycoplasma bovis has also been cultured from blood.11 There are studies in which M. Received for publication October 7, 1994. bovis was inoculated into calves concurrently with Pasteurella hemolytica, 3 respiratory syncytial virus, 11 and bovine viral diarrhea virus and P. hemolytica. 8 Lesions produced by M. bovis alone were described as focal areas of coagulative necrosis surrounded by mononuclear cells and suppurative bronchiolitis with varying degrees of lympho...