The first general method for the selection of boronic acid-based aptamers (boronolectins) that allows for glycan substructure focusing is described. Using fibrinogen as a model glycoprotein, we have selected boronic acid-modified DNA aptamers that have high affinities (low nM K-d) and the ability to recognize changes in the glycosylation site. The method developed should also be applicable to the development of aptamers for other glycoproducts, such as glycolipids and glycopeptides.