Epidemiology has suggested that elderly populations may be more susceptible to respiratory infections and the deleterious effects of specific inhaled particles. The objective of the present study was to examine the effect of advanced age on acute mortality and pulmonary clearance following exposure to a bacterial pathogen. Male Fischer 344 rats, either 10 weeks or 20 months of age, were intratracheally inoculated with 5.0 x 10^5 Listeria monocytogenes. Acute mortality was assessed for 7 days following instillation. To assess bacterial clearance at 3 days post-instillation, homgenized left lungs were cultured and colony-forming units were counted following an overnight incubation. Lung macrophages were collected by bronchoalveolar lavage from right lungs, and chemiluminescence (CL), an index of macrophage activation, was measured. At 5 and 7 days post intstillation, 90% and 20% of the 10-week rats were still alive, respectively. In contrats (p<0.05), only 20% of the 20-month old rats had died by 7 days. At 3 days post-instillation, there were 2.5 times as many L. monocytogenes remaining in the lungs of the 20-month old rats as compared to the 10-week rats, and macrophage CL was 57% lower in the aged rats. The study demonstrated that advanced age is associated with increased susceptibility to pulmonary bacterial infection marked by elevated mortality, slowed pulmonary bacterial clearance, and suppressed macrophage function. These observations are indicative of reduced pulmonary defense function in an older population of rats.