We have isolated a human lectin with specificity for terminal Gal and GalNAc residues by affinity chromatography on p-aminophenyl-β-D-thiogalactosyl Sepharose. The human protein consists of a single subunit (Mcr) 41,000) as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The amino acid composition closely resembles that of the A and B subunits of the rabbit hepatic β-galactoside-specific binding protein whereas the carbohydrate composition more closely approximates that of the A subunit. Binding of asialoglycoproteins with terminal galactose residues is readily reversible with a dissociation rate for 125 I-asialo-orosomucoid of 1.7 x10-3 s-1. Inhibition constants (k (i)) were determined for a number of asialoglycoproteins, glycopeptides with complex oligosaccharides having 1 to 3 terminal Gal residues, and glycopeptides bearing from 1 to 24 O-glycosidically linked Galβ1, 3GalNAc units or GalNAc residues. Removal of the Gal residues from Galβ1, 3GalNAc units results in a 10-to 27-fold reduction in the k (i) values for glycopeptides bearing these moieties. Thus the avidity of the human carbohydrate binding protein is considerably greater for terminal GalNAc residues than Gal residues. Glycopeptides with complex oligosaccharides bearing three terminal Gal residues have a 30-fold lower k (i) than those with two terminal Gal residues. A similar reduction in k (i) was observed for glycopeptides bearing four Galβ1, 3GalNAc or GalNAc units as compared to those bearing two such units. We suggest that this effect is most likely due to simultaneous binding at two sites rather than to a statistical effect of increased numbers of residues. If this is the case, the binding sites would appear to be within 25 to 30 Å of each other.