Functionally important glycosylation has been identified in the antigen binding domain of an anti-GnRH monoclonal antibody. Presence of mannose and sialic acid residues is revealed from con A immunoblots and positive staining with a sialic acid detection kit, respectively. Desialylation of the antibody reduces GnRH binding, suggesting the role of terminal sialic acid residues in modulating antigen binding. The crystal structure of the Fab fragment shows electron density adjacent to the antigen binding site which may be attributed to the covalently attached carbohydrate moiety. Thus, the presence of sialic acid containing mannose-rich carbohydrate moiety near the antigen binding site of a monoclonal antibody Fab fragment is relevant for defining antibody specificity.