The effects of the infection by Mycoplasma dispar were determined under in vivo conditions by measuring the tracheobronchial clearance of Sarratia marcescens in calves before and after infection with this mycoplasma species. Quantitative culture was done on tracheobronchial samples obtained from calves at various times after the inoculation of S. marcescens. Clearance was essentially complete by 4 hours in calves free of M. dispar. In contrast, the clearance of S. marcescens in the same animals 5 days after intratracheal exposure with M. dispar showed that it was extended beyond 4 hours. Scanning electron microscopy and histopathological studies of infected lungs indicated that M. dispar infection resulted in areas of degeneration and destruction of the ciliated epithelium. These lesions may be responsible for the altered clearance observed in calves infected with M. dispar;The capacity of encapsulated or unencapsulated M. dispar to avoid phagocytosis, to impair bactericidal activity, and to activate bovine alveolar macrophages (BAM) was also investigated. Live or heat inactivated encapsulated and unencapsulated M. dispar or purified capsule were cultured with BAM, in the presence or absence of antibodies against M. dispar or purified capsule anticapsular serum. Phagocytosis occurred in presence of anticapsule antibodies. Bovine alveolar macrophages were treated with encapsulated and unencapsulated M. dispar or purified capsule and tested for their killing activity on Staphylococcus aureus or Serratia marcescens, production of TNF and IL-1, and consumption of glucose. Killing of both bacterial species was reduced in BAM exposed to purified capsule or capsulated mycoplasma but not when unencapsulated M. dispar was used. Reduced production of TNF and IL-1 and low glucose consumption was seen in BAM treated with encapsulated mycoplasmas or purified capsule, whereas unencapsulated M. dispar induced TNF and IL-1 production. In conclusion, while M. dispar infection produces lesions of the ciliated epithelium of the lower respiratory tract associated with reduction in the respiratory clearance of bacteria, the capsule of the mycoplasma also exerts suppressive effects on BAM.