In order to investigate the phenolic compounds inducing the expression of Ti-plasmid vir genes of Agrobacterium tumefaciens KU12, we tested whether the eighteen phenolic compounds known as vir gene inducer have the activity to induce vir operon of the Ti-plasmid in A. tumefaciens KU12 transformed with pSM358cd. Also, the phenolic compounds in some tumor-uninduced dicotyledons and the attachment ability of a. tumefaciens KU12 on such dicotyledonous plants were investigated in an effort to analyze the reason why no tumor is formed on the plants. As results, fifteen among eighteen phenolic compounds known as vir gene inducer induced the expression of the Ti-plasmid vir genes. Some dicotyledonous plants which do not form the tumor have the phenolic compounds inducing the vir genes, but have less ability of the attachment than the dicotyledonous plants forming tumor.