The in vitro antibacterial activity of rat alveolar macrophages against a challenge of radiolabeled Staphylococcus epidermidis was studied following 30 and 60 consecutive days of in vivo tobacco smoke inhalation in a dose equivalent to approximately 1 1/2 packs of unfiltered cigarettes per day in man. Macrophages harvested by bronchopulmonary lavage, were cultured, infected, and assayed hourly for 3 hr to determine the relative percentage of surviving radiolabeled intracellular bacteria. Macrophages harvested from smoke-treated rats for 30 days had an impaired capacity to kill bacteria when compared to macrophages from control and sham-smoked animals (63.5% +/- 3.3, 80.8% +/- 3.8, and 86.2% %/- 1.2 killed in 3 hr, respectively). Alveolar macrophages harvested from rats exposed to cigarette smoke for 60 days, however, did not have an impairment in their bactericidal activity relative to matched controls (73.3% +/- 3.9 vs 69.8% +/- 6.9 killed in 3 hr). These data imply that doses of cigarette smoke commonly consumed by man impair the in vitro bactericidal function of the rat alveolar macrophages following 30 consecutive days of experimental smoke treatment. The disappearance of this effect after 60 days of exposure to tobacco smoke suggests that an adaptation to the initial impairment has developed.