Rhizobium rhizogenes, a soil bacterium, is the causative agent of the neoplastic disease hairy root. Upon incubation of Rhizobiuni rhizogenes A4 with coniferyl alcohol, a lignin precursor, bacterial virulence on cotton cotyledon slices was stimulated. This was observed both in numbers of root hairs produced and in their length. Stimulation was maximized after exposure of bacteria to 150 mu g/mL of coniferyl alcohol for 4 h. This was shown to be at the early log phase of bacterial growth.