Surfactant protein D (SP-D) is a pulmonary collectin, containing collagen and lectin domains, important in innate host defense in the lung. It agglutinates and opsonizes bacteria and leads to phagocytosis by macrophages. Matrix metalloproteinase-9 (MMP-9) is a gelatinase upregulated in many inflammatory diseases of the lung. We tested SP-D as a substrate for MMP-9, and assessed SP-D function in this context. We show cleavage at three discrete sites of the collagen domain, generating C-terminal fragments of 34, 27, and 19KDa identified by western blot and with specific cleavage sites verified by mass spectrometry. SP-D is not, however, hydrolyzed by the related protease MMP-8, nor the serine protease prolyl endopeptidase, both of which cleave collagen. Incubation with these three proteases separately or in combination did not generate any detectable quantity of the neutrophil chemoattractant proline-glycine-proline (PGP), though SP-D contains many PGP motifs in the collagen domain. Furthermore, with MMP-9 cleavage, SP-D's intrinsic neutrophil chemoattractant activity was eliminated and its ability to aggregate Pseudomonas aeruginosa was also decreased. These studies indicate that in inflammatory lung diseases where MMP-9 is upregulated, impairment of SP-D's innate immunologic properties may increase frequency and severity of pulmonary infection, emphasizing the role of MMP-9 in disease.