This work involved (1) the identification of mollicutes and other microorganisms associated with infectious respiratory disease of calves and (2) an in vitro investigation into the effects of Mycoplasma bovis and M. bovoculi on fetal bovine tracheal explant cultures (TEC). Approximately 90% of the pneumonic calf lungs examined yielded microorganisms (bacteria 74%, mollicutes 63%, viruses 43%). Pasteurella multocida 39%, P. haemolytica 31% and H. somnus 12% were the most frequent bacteria cultured. Mycoplasma dispar 40%, M. bovis 34%, Ureaplasma diversum 24% and M. bovirhinis 7% were isolated from the lungs: M. arginini, M. bovigenitalilum and acholeplasmas were not detected. Noncytopathic bovine viral diarrhea (BVD) virus 27% and bovine herpesvirus-1 (BHV-1) 16% were the most common viruses isolated. The interaction between M. bovis or U. diversum and P. multocida was highly significant (P =.0001 and.0006) leading to the suggestion that the 2 mollicutes could possibly predispose calves to P. multocida infection.;Immunofluorescent (IF) examination of cryostat sections of lung tissues revealed that M. dispar preferentially colonized ciliated epithelium whereas in addition to colonizing ciliated epithelium, M. bovis also was detected in the lung parenchyma. Also, M. dispar remained viable in tissue held at 4 C for 14 days; whereas the organisms could be detected by IF in the same tissues for up to 7 weeks.;Both M. bovis and M. bovoculi replicated in TEC but only M. bovis produced complete ciliostasis which was dependent on serum. Addition of catalase to the explant cultures did not inhibit ciliostasis; also, formaldehyde-killed organisms did not affect TEC ciliary activity. Mycoplasma bovis was observed by IF in association with ciliated epithelium in only 2 instances. A transition from ciliated columnar to non-ciliated squamous epithelium was seen in thin sections of explants, suggesting that a toxin may have been associated with the ciliostasis. However, attempts to demonstrate a soluble toxin in sterile spent TEC medium were unsuccessful. It is concluded that in the presence of serum, viable M. bovis organisms produce a factor(s) which induced ciliostasis in fetal bovine TEC. This model should serve as means for further studies on the virulence mechanisms of M. bovis and other mollicutes associated with bovine infectious respiratory disease.