To study the change in intrapulmonary bacterial growth rate over time during Gram-negative pneumonia, a two-hit model of recurrent bacterial aspiration was developed in mice. A mutant of K. pneumoniae was isolated that could be distinguished from the wildtype, when cultured on appropriate media. These two strains were intranasally administered, 4 hours apart, to mice whose lungs were quantitatively cultured 24 hours later. The relative burden of each aspirated inoculum was determined, and, using the administered dose and the number of bacteria from each inoculum present at the end of the experiment, first-order growth constants for each inoculum were also calculated. Results indicate that following an initial aspiration of this organism, subsequently aspirated bacteria proliferate more slowly. When two aspirations occurred 4 hours apart from one another the bacteria aspirated first represented 96% of total lung burden at 24 hours. The growth constant of the second inoculum was related to the magnitude of the first inoculum in an inverse, nonlinear fashion. When parallel experiments were performed in complement C3 deficient mice no suppression of the second inoculum was noted, suggesting that early upregulation of antibacterial activity in the lung is a C3-mediated event.