A FAMILY of G-protein-eoupled chemoattractant receptors is known to mediate the transport and activation of neutrophils and macrophages. This family includes receptors for chemokines, such as interleukin-8, bacterial formylated peptides, platelet-activating factor, leukotriene B4, and the complement anaphyla-toxins^1–3. The apparent redundancy of these receptors suggests that they have an important underlying role in host defence. To isolate the contribution of particular molecules, we disrupted a gene that encodes a single chemoattractant receptor. Here we show that mice deficient in the chemoattractant C5a receptor, in comparison to their wild-type littermates, were unable to clear intrapulmonary-instilled Pseudomonas aeruginosa, despite a marked increase in neutrophil influx, and succumbed to pneumonia. These C5a-receptor-deficient mice challenged with sub-lethal inocula of Pseudomonas become superinfected with secondary bacterial strains. We conclude that the C5a receptor has a non-redundant function, and is required for mucosal host defence in the lung.