An expression vector containing the Rhizobium leguminosarum nodA promoter cloned in front of the Escherichia coli lacZ gene was used to characterize the properties of the R. leguminosarum nodA gene-inducing compound(s) present in sterile root exudate of the host plant Vicia sativa L. subsp. nigra (L.). The major inducing compound was flavonoid in nature, most likely a flavanone. The commercially available flavonoids naringenin (5,7,4'-trihydroxyflavanone), eriodictyol (5,7,3'4'-tetrahydroxyflavanone), apigenin (5,7,4'-trihydroxyflavone), and luteolin (5,7,3',4'-tetrahydroxyflavone) induced the nodA promoter to the same level as the root exudate. On the basis of chromatographic properties, it was concluded that none of these compounds is identical to the inducer that is present in root exudate. The induction of the nodA promoter by root exudate and by the most effective inducer naringenin was very similar, as judged from the genetic requirements and the kinetics of induction.