Monoclonal antibodies (mAbs) against a plant NADPH-cytochrome P-450 (Cyt P-450) reductase from Jerusalem artichoke (Helianthus tuberosus) tuber were prepared. These antibodies were produced by hybridoma resulting from the fusion of spleen cells from a rat immunized with a purified preparation of the reductase and mouse myeloma cells. The mAbs thus obtained were screened for their interaction with the reductases, first in western dots and then in blots, and for their ability to inhibit the NADPH-cytochrome c (Cyt c) reductase activity from Jerusalem artichoke microsomes. Among the 11 clones giving a positive response on western blots, only 6 were also able to inhibit microsomal NADPH-Cyt c reductase activity, and the microsomal Cyt P-450 monooxygenase activities dependent upon electrons transferred by the reductase. Thus, two families of mAbs were characterized: a family of mAbs that interact with epitopes of the reductase implicated in the reduction of Cyt P-450 by NADPH (binding sites for NADPH, flavin mononucleotide, flavin adenine dinucleotide, and Cyt P-450), and a structural family, whose members recognize epitopes outside the active site of the reductases. These mAbs specifically recognize the reductase, and all of them interact with all of the isoforms, indicating that important primary or secondary structural analogies exist between the isoforms, not only at the active site, but also at the level of epitopes not directly associated with catalytic activity.