A plant flavone, luteolin is a well-known inducer of nod genes in the Rhizobium meliloti. Its poor aqueous solubility was greatly enhanced by the complexation with a family of cyclosophoraoses synthesized in R.meliloti. Nuclear magnetic resonance (NMR) spectroscopic analysis showed that the chemical shifts of the aromatic ring moieties of the luteolin were changed greatly by the complexation with cyclosophoraoses. Fourier transform infrared (FTIR) spectroscopic analysis also showed a restricted vibrational pattern in carbonyl stretching region of the luteolin due to the complexation. This effective complex formation of cyclosophoraoses with a plant flavone, luteolin, suggests that rhizobial cyclosophoraoses play an important role as a solubility enhancer of the hydrophobic legume-derived flavonoids.