Lectins were evidenced on the surface of one Agrobacterium tumefaciens wild strain (82,139) by agglutination test and neoglycoprotein labelling. Bacteria were incubated in the presence of various fluorescein-labelled neoglycoproteins and the binding was assessed by a fluorimetric method. Among the fluorescein-labelled neoglycoproteins tested, the one bearing alpha-D-galactosyl residues was the most efficient. The labelling was optimal at pH 5.0 and naught at pH above 7. The binding was specifically inhibited by homologous fluorescein-free neoglycoproteins. A galactoside-specific lectin was purified to homogeneity by affinity chromatography on agarose-A4 substituted with alpha-D-galactopyranosyl residues. Upon polyacrylamide gel electrophoresis, a single band (M(r) 58,000) was detected. This alpha-D-galactoside-specific lectin agglutinated preferentially human B red blood cells at pH 5.0. Another lectin specific for alpha-L-rhamnoside (M(r) 40,000) not retained on the immobilised galactose was purified by affinity chromatography on alpha-L-rhamnosyl substituted agarose-A4. This L-rhamnoside-specific lectin preferentially agglutinated horse erythrocytes. On the basis of their M(r) and on their sugar specificity, these two lectins are novel lectins with regard to the known sugar-binding proteins present in the Rhizobiaceae family: Agrobacterium, Rhizobium or Bradyrhizobium strains.