Ficolins, which are comprised of a collagen-like domain and a fibrinogen-like domain, are a kind of pattern-recognition molecule for pathogens in the innate immunity system. To investigate the molecular mechanism of the discrimination between self and non-self by ficolins, human M-ficolin fibrinogen-like domain (FD1), which contains the ligand-binding site, was overexpressed in Pichia pastoris, purified and crystallized using the vapour-diffusion method at 293 K. The crystals belong to the monoclinic space group P2(1), with unit-cell parameters a = 55.16, b = 117.45, c = 55.19 angstroms, beta = 99.88 degrees, and contain three molecules per asymmetric unit. An X-ray data set was collected to 1.9 angstroms resolution using synchrotron radiation at beamline BL24XU at the SPring-8 facility in Japan.