Multiple intranasal experimental Mycoplasma (M.) bovis infection was induced to 22 calves and resulted in clinical manifestations. M. bovis was re-isolated from 17 of 24 lungs which had been altered by pneumonia. Pasteurella haemolytica was the only pathogen isolated from the pathologically affected lung of one of the experimental animals. The haematogenic proliferation phase was successfully identified in 5 of 8 examined calves. Arthritis was not recordable from any of the experimental animals. Interstitial and catarrhal to catarrhal-purulent bronchopneumonias were the most common histological findings. Peribronchial lymphoid cell accumulations occurred frequently, as time passed from onset of infection. Antibodies against M. bovis were detected in blood serum of infected animals by means of film inhibition, as of the 3. week from infection, and by means of indirect haemagglutination, as of the 4. week from infection.