M-ficolin, which forms trimer-based multimers, is a pathogen-recognition protein in the innate immune system, and it binds to ligands through its fibrinogen-like (FBG) domain. As the first step toward the elucidation of the molecular basis for pathogen-recognition by the M-ficolin multimers, we assigned the backbone resonances of the monomeric mutant of the M-ficolin FBG domain, recombinantly expressed by Brevibacillus choshinensis. Like the wild-type trimeric FBG domain, the monomeric FBG domain also requires His251, His284 and His297 for the ligand-binding activity, as judged by mutational analyses using zonal affinity chromatography. The secondary structure predicted by the backbone resonance assignments is similar to that of the trimeric FBG domain in the crystal, indicating that the monomeric FBG domain is folded correctly to perform its function.