Gibberellins, a class of plant hormones. consist of more than 120 members. Only a few of them are recognized by a receptor that remains unknown. The haptenic mouse monoclonal antibody. 4-B8(8)/E9, was generated against gibberellin A(4)(GA(4)) to recognize biologically active GA selectivity, and we attempted to confirm the binding properties between the antibody and GA4. We carried out an X-ray crystallographic analysis of the 4-B8(8)/E9 Fab fragment complexed with GA4 at a 2.8 A resolution by using the molecular replacement method. The crystal structure of the Fab fragment showed the typical immunoglobulin fold of the P-barrel structure which is the common motif of all antibodies. A small hapten-combining site was made up of three heavy chain CDR loops. On the other hand, CDRs of the light chain did not interact directly with GA(4). The C/D rings of the GA(4) molecule were in van der Waals contact mainly with the aromatic side chain of Tyr100AH and Phe100BH of CDR-H3, The 3beta-hydroxyl and 6beta-carboxyl groups were. respectively. hydrogen-bonded to the main chain of Ala33H and to the Thr53H heavy chain. (C) 2002 Elsevier Science (USA). All rights reserved.