Wyerone is the predominant phytoalexin produced by Vicia faba. At a concentration of 100-mu-M, wyerone prolonged the lag phase before the onset of exponential growth of different strains of Rhizobium leguminosarum and Bradyrhizobium japonicum. The response to wyerone was dose dependent, with a shorter lag phase occurring at lower concentrations. Wyerone was only moderately inhibitory towards the phytopathogenic bacterium Erwinia carotovora cv. atroseptica. HPLC analysis of the medium during bacterial growth indicated that Rhizobium leguminosarum was able to metabolize wyerone. The identification of the product as hydroxyester wyerol was confirmed by GC/MS analysis.