Collectins, C-type (Ca2+-dependent) animal lectins with both collagenous and carbohydrate recognition domains, function as opsonins against pathogens. We previously described an N-acetylglucosamine (GlcNAc)-binding lectin (ficolin/P35) with a collagen- and a fibrinogen-like sequence present in human serum. In this report we show that ficolin/P35 can serve as an opsonin and enhance the clearance of pathogens having surface GlcNAc. Ficolin/P35 bound to an Ra chemotype strain of Salmonella typhimurium (TV119) which has an exposed GlcNAc at the non-reducing termini of the polysaccharide. On the other hand, ficolin/P35 did not bind to LT2, a smooth type strain of S. typhimurium with additional O-polysaccharides covering GlcNAc. Ficolin/P35 enhanced the uptake of TV119 by monocytes or polymorphonuclear leukocytes but had no opsonic activity towards LT2. These results suggest that, like collectins, ficolin/P35 is a collagenous lectin which has a role in innate immunity against certain pathogenic organisms by acting as an opsonin. We prepared monoclonal antibodies against ficolin/P35 and developed an enzyme-linked immunosorbent assay (ELISA) for measuring ficolin/P35 concentrations in humans. The mean serum concentration of ficolin/P35 from 130 normal individuals was estimated to be 13.7 microg/ml.