We have modeled the auxin-binding site of maize auxin-binding protein 1 (ABP1) using Site I binding data determined by Ray et al. [(1977) Plant Physiol. 60, 585] for 45 different auxin analogues. We propose an auxin phytophore model based on the structures of the three compounds which bind with the highest affinity, naphthalene-1-acetic acid, naphthalene-2-acetic acid, and indole-3-acetic acid. We propose that their phytophoric conformation is one in which the planes described by their carboxylic acid groups are perpendicular to the plane of their aromatic ring systems and their carboxylic acid oxygens are positioned directly astride the ring system. Our model of the auxin-binding site describes three regions of the protein: a planer aromatic ring binding platform, a carboxylic acid binding site, and a hydrophobic transition region which separates the two binding sites. The exact orientation and position of the ABP1 groups involved in carboxylic acid binding is somewhat flexible. This is the first model proposed which describes the auxin-binding site of a single auxin-binding protein. We have used this model to propose a number of compounds which should bind ABP1 well.