A lectin that displays specificity for the core region of asparagine-linked oligosaccharides (Man3GlcNAc2-Asn) was isolated from human serum and liver by affinity chromatography on mannan-Sepharose. The designation 'core-specific lectin' (CSL) is used to indicate its specificity. Selective elution of human CSL from mannan-Sepharose was accomplished with 50 mM-mannose. Two additional proteins that displayed Ca2+-dependent binding to mannan-Sepharose were eluted by mannose 6-phosphate or β-glycerophosphate but not by mannose. The latter proteins were identified as C-reactive protein and serum amyloid protein. Human CSL isolated from liver was indistinguishable from serum CSL in its physicochemical properties, immunological properties and specificity. The N-terminal sequence of human CSL is homologous to that reported for 'mannan-binding protein C' (MBP-C) [Drickamer, Dordal & Reynolds (1986) J. Biol. Chem. 261, 6878-6887]. The amino acid composition of human CSL is similar to that of rat MBP-C, including the presence of hydroxyproline and hydroxylysine residues. Collagen-like sequences with hydroxylated proline and lysine residues appear to be present in human CSL as well as in rat CSL. The collagen-like regions of human and rat CSL may play a role in assembly of CSL subunits into complexes consisting of nine subunits that display Ca2+-dependent carbohydrate-binding activity.