Mouse plasma ficolin was purified by GlcNAc affinity and anion-exchange chromatography. Gel-filtration chromatography and gradient sedimentation indicated that mouse plasma ficolin is a 12-mer of approximately 35 kDa subunits, and electron microscopy showed the same parachute-like structure previously characterized for the pig ficolin 12-mer. Whereas the predominant form in pig plasma is a 24-mer, mouse and human plasma ficolin showed only the 12-mer form. We conclude that mouse plasma ficolin corresponds to the recently described ficolin A. We have identified a second mouse ficolin gene, ficolin B, which means that pig and mouse each have two ficolin genes, and human has three. One ficolin gene in all species is expressed in liver and is the primary source of plasma ficolin. Expression of this gene in other tissues, and expression of the second ficolin gene, appears to vary in different species.