An auxin-binding protein was purified from membranes of maize shoots including the coleoptiles, leaf rolls and mesocotyls. The method of Ca2+-promoted sedimentation of membrane particles was adopted for large-scale preparation. The auxin-binding protein was solubilized from the acetone-washed membranes, and purified by successive chromatographies on DEAE-Sephacel, 1-naphthylacetic acid-linked AH-Sepharose 4B, and Sephadex G-100 columns. The yield of the purified protein was about 0.2 mg from 1 kg of shoots. The binding protein exists as a dimer with a subunit molecular weight of 21,000, and possesses one auxin-binding site per dimer. The preparation also contains a minor molecular form with a subunit molecular weight of 20,000. The auxin-binding protein is not a hydrophobic protein, as judged from its amino acid composition and solubility. The circular dichroic (CD) spectrum of the binding protein resembles the spectrum anticipated from the beta-structures, and shows no alpha-helix characteristic in the secondary structure. The CD spectral changes induced on the binding of auxin and its antagonist seem to be related to the receptor function. The affinity of the binding protein for auxin is dependent on pH, with an optimum at pH 5.0, while the binding protein is unstable below pH 6. We discuss here the intracellular localization of the auxin-binding protein from the view point of the controversial pH-dependence of the binding affinity and stability.