Macrophage inflammatory protein 1α (MIP-1α) (CCL3) is an important mediator of leukocyte recruitment and activation in a variety of inflammatory states, including infection. A recombinant human type 5 adenovirus containing the murine MIP-1α cDNA (AdMIP-1α) was constructed to determine the effect of transient intrapulmonary expression of MIP-1α on leukocyte recruitment, activation, and bacterial clearance in a murine model of Klebsiella pneumoniae pneumonia. The intratracheal administration of AdMIP-1α resulted in both time- and dose-dependent expression of MIP-1α mRNA and protein within the lung. Importantly, the intrapulmonary overexpression of MIP-1α resulted in a maximal 35- and 100-fold reduction in lung and blood bacterial burden, respectively, in animals cochallenged with K. pneumoniae, which was associated with a significant increase in neutrophil and activated NK cell accumulation. Furthermore, the transgenic expression of MIP-1α during bacterial pneumonia resulted in enhanced expression of gamma interferon mRNA, compared to that observed in Klebsiella-challenged animals pretreated with control vector. These findings indicate an important role for MIP-1α in the recruitment and activation of selected leukocyte populations in vivo and identify this cytokine as a potential immunoadjuvant to be employed in the setting of localized bacterial infection.