NIH/S, Swiss Webster, and BALB/c mice were infected intranasally with three Chlamydia pneumoniae isolates, Kajaani 6, Helsinki 12, and TW-183. C. pneumoniae could be isolated from the lung homogenates and bronchoalveolar lavage fluids up to the third week post-infection. Specific serum IgG antibodies against C. pneumoniae reached high levels in the third week and remained elevated until the end of the 6-week follow-up period. Serum IgM levels were highest in the third week post-infection and started to decrease thereafter. In spite of these signs of ongoing infection, the mice did not show any symptoms of disease. NIH/S mice could be readily and uniformly infected, while BALB/c mice were the most resistant and developed the weakest antibody response. The greatest histological changes were detected in NIH/S mice as well. The inflammatory infiltrate, which consisted of lymphocytes and plasma cells throughout the study, was restricted to the peribronchial and perivascular space and to the interstitium of the lung parenchyma.