The exposed carbohydrate residues on the cell surface of both tumourigenic and nontumourigenic strains ofAgrobacterium have been investigated using lectins as probes. N-acetyl-D-galactosamine and β-D-galactose were found to be present as the exposed groups on the cell surfaces of heAgrobacterium strains. These carbohydrate residues are attached to lipids on the outer membrane of the bacteria as lipopolysaccha-rides. Fluorescently labelled lectins were used to observe bacterial agglutination in fluorescence microscope. The involvement of these exposed carbohydrate groups in host-pathogen interaction was demonstrated by actual inhibition of tumour initiation at wound sites on host plant.