A β-D-lactose ligand equipped with a terminal carboxylic acid through a hydrophilic PEG4 spacer. The carboxylic acid can be activated to NHS, sulfo-NHS, or pentafluorophenyl esters and reacted with amine-functionalized platforms or lysine residues on proteins and peptides to form amide bonds. This compound provides a lactose (Galβ1-4Glc) building block for glycoconjugate synthesis and the construction of probes for galactose-recognizing proteins, including galectins and the hepatic asialoglycoprotein receptor; it is a substrate for sialyltransferases and other glycosyltransferases.