Saturation transfer difference (STD) NMR spectroscopy is a promising tool for rapid screening, identifying ligands that interact with a target protein, and characterizing the epitopes of the ligands. Gibbrellins (GAs) are a class of plant hormones and form a large family consisting of more than 120 members. A few of them, called "active" GAs, are considered to be perceptible to a receptor that remains unknown. We applied STD NMR spectroscopy to detect the binding activity and identify the binding epitope of gibberellin A(3) (GA(3)) that is recognized by monoclonal antibody 4-B8(8)/E9. This is one of the antibodies that can mimic a GA receptor in the manner of recognition of active GAs. The information on the binding epitope, obtained by STD NMR, was in good agreement with that shown by analyzing the crystal structure of the antibody-GA(4) complex. This suggests that STD NMR spectroscopy would be very useful to characterize the interaction between GAs and such binding proteins as GA-catabolic enzymes and receptors. (C) 2003 Elsevier Inc. All rights reserved.