Asthma is a major cause of morbidity worldwide with prevalence and severity still increasing at an alarming pace. Hallmarks of this disease include early-phase bronchoconstriction with subsequent eosinophil infiltration, symptoms that may be mimicked in vivo by the complement-derived C3a anaphylatoxin, following its interaction with the single-copy C3aR. We analyzed the pathophysiological role of the C3a anaphylatoxin in a model of experimental OVA-induced allergic asthma, using an inbred guinea pig strain phenotypically unresponsive to C3a. Molecular analysis of this defect revealed a point mutation within the coding region of the C3aR that creates a stop codon, thereby effectively inactivating gene function. When challenged by OVA inhalation, sensitized animals of this strain exhibited a bronchoconstriction decreased by∼ 30% in comparison to the corresponding wild-type strain. These data suggest an important role of C3a in the pathogenesis of asthma and define a novel target for drug intervention strategies.