Forty-eight hysterectomy produced piglets reared in a pathogen-free environment were housed in 4 separate units. Three groups were intratracheally challenged with M hyopneumoniae at 2 wk, or at 16 wk, or at 2 and 16 wk of age. The fourth group was uninfected and received broth medium. Coughing started 2 weeks after the first infection but the booster infection did not induce any coughing. Macroscopic lesions, typical of mycoplasmal pneumonia, were noted 1 wk post-infection. After 7-9 wk, piglets showed recovering lesions. In the group that was twice infected, only 1 pig had extensive pneumonia after the second infection. The remaining pigs only had recovering lesions (RL) (resulting from the first challenge). Histopathology and scanning electron microscopy confirmed the macroscopic observations. Antibodies were detected by ELISA and immunoblotting at 3-4 wk post-infection, peaking after 11-12 wk and then gradually decreasing. However, the antibody response increased after the second infection. These findings showed a noticeable recovery following infection with M hyopneumoniae and a clear resistance to pneumonia following the booster infection.