Virulence (vir) gene expression of Agrobacterium tumefaciens is activated by plant phenolic compounds such as α-hydroxyacetosyringone (HOAS), acetosyringopone (AS), methyl syringate, coniferyl alcohol and sinapyl alcohol.Inositol was found to be a potentiating factor of vir-inducing activity, which enhanced the vir-inducing activity of AS and HOAS in a synergistic manner, in particular at a low concentration of AS and HOAS. Of the other sugars tested D-gluose, L-rhamnose, D-xylose and D-galacturonic acid, the main components of plant cell wall polysaccharides, remarkedly potentiated the vir-inducing activity of AS, indicating the cooperative action of the signal compunds and sugars in Agrobacterium infection to plants.