We have used the analytical system based on surface plasmon resonance to monitor the interaction between Amaranthus hypochondriacus var. Mexico lectin and four different fetuins; fetuin, asialofetuin, agalactofetuin, and agalactosaminofetuin. Agalactofetuin and agalactosaminofetuin were prepared by enzymic digestion of asialofetuin using jack bean beta-galactosidase or endo-alpha-N-acetylgalactosaminidase from Diplococcus pneumoniae. Ligands were immobilized onto a sensor surface via amide linkages. The lectin interacted most strongly with asialofetuin, but not with agalactosaminofetuin. The binding of the lectin to asialofetuin was inhibited by N-acetylgalactosamine or Gal beta 1-->3GalNAc in a dose-dependent manner.