SP-D (surfactant protein-D) is a 43?kDa member of the collectin family of innate immune modulators. It is constitutively secreted by alveolar lining cells and epithelium associated with tubular structures. Its principal components consist of a collagen-like region and a C-terminal carbohydrate recognition domain (CRD), a structure that further places it in a subset of an expanded group of proteins termed defense collagens. SP-D is known to bind both SIRP alpha and the calreticulin/CD91 complex on macrophages. When the ratio of antigen/pathogen to available CRDs is low, antigen can be bound without occupying all available CRDs. The free CRDs will bind to SIRP alpha, generating a signal that downmodulates the inflammatory response. When virtually all CRDs are occupied by ligand, however, free CRDs are not available for SIRP alpha binding. The activity of the recombinant rat SPD was measured by its ability to bind fluorescein-conjugated?E. coli bioparticles. The rrSPD was diluted to 10 ug/ml in assay buffer of 20 mM Tris, 137 mM NaCl, 1 mM CaC12, pH7.4 and fluorescein-conjugated?E. coli was diluted to 7*108 cells/ml. Equal volume of 10 ug/ml rrSPD and fluorescein-conjugated?E. coli were mixed and incubated at room temperature for 1h. 10 ul mixture was took onto the slide and observed under the fluorescence microscopy. The result was shown in figure 1, the rrSPD could bind fluorescein-conjugated?E. coli bioparticles.