The incidence of hypersensitivity pneumonitis (HP) is lower in smokers than in nonsmokers. Because nicotine is immunosuppressive, we hypothesized that it could have a protective effect on HP induction in vivo. HP was induced in mice that were treated with nicotine either intraperitoneally (IP) (0.5 to 2.0 mg/kg/day) or intranasally (IN) (0.025 to 2.0 mg/kg/day). Both IP- and IN-treated animals had fewer bronchoalveolar lavage total cells and lymphocytes and a decreased lung tissue inflammation. IFN-gamma but not interleukin-10 mRNA expression was reduced in lung tissue of 2.0-mg/kg IN-treated animals. To test the effect of nicotine on alveolar macrophages, AMJ2-C11 cells were treated with nicotine and stimulated with lipopolysaccharide or Saccharopolyspora rectivirgula, a causative agent of HP. Nicotine reduced tumor necrosis factor release and tumor necrosis factor, interleukin-10, and IFN-gamma mRNA expression after stimulation and decreased CD80 expression by 55% in lipopolysaccharide-stimulated cells and by 41% in S. rectivirgula-stimulated cells. We conclude that nicotine could be, at least in part, responsible for the protection observed in smokers against HP. The inhibitory effect of nicotine on alveolar macrophages could be one of the mechanisms involved.