Leukotrienes (LTs) are considered important for antibacterial defense in the lung. Multidrug resistance protein 1 (mrp1) is a transmembrane protein responsible for the cellular extrusion of LTC(4). To determine the role of mrp1 in host defense against pneumonia, mrp1(-/-) and wild-type mice were intranasally inoculated with Streptococcus pneumoniae. mrp1(-/-) mice displayed a diminished outgrowth of pneumococci in lungs and a strongly reduced mortality. These findings were related to an effect of mrp1 on LT metabolism, because survival was similar in mrp1(-/-) and wild-type mice treated with the 5-lipoxygenase-activating protein inhibitor MK-886. Although LTC(4) levels remained low in the bronchoalveolar lavage fluid of mrp1(-/-) mice, LTB(4) concentrations were higher than in wild-type mice. These elevated LTB(4) concentrations were important for the relative protection of mrp1(-/-) mice, because the LTB(4) antagonist LTB(4)-dimethyl amide abolished their survival advantage. In vitro experiments suggested that the intracellullar accumulation of LTC(4) in mrp1(-/-) mice results in product inhibition of LTC(4)-synthase, diminishing substrate competition between LTA(4)-hydrolase (which yields LTB(4)) and LTC(4)-synthase for the available LTA(4). We conclude that mrp1(-/-) mice are resistant against pneumococcal pneumonia by a mechanism that involves increased release of LTB(4). These results identify mrp1 as a novel target for adjunctive therapy in pneumonia.