When ,B-indolylacetic acid (IAA) (10-5M) is added to isolated flax roots, lateral roots are induced to form adventitiously. If some lateral root initials are already present when IAA is added, newly induced primordia form preferentially immediately adjacent to them. It is suggested that a chemical stimulus, a kinin, originating from the existing root meristems interacts with IAA to induce further cell division in the pericycle. It is proposed that kinin formation may be a normal accompaniment of cell division.