The three-dimensional structure of a novel Kunitz (STI) family member, an inhibitor purified from Delonix regia seeds.(DrTI), was solved by molecular replacement method and refined, respectively, to R-factor and R-free values of 21.5% and 25.3% at 1.75 Angstrom resolution. The structure has a classical beta-trefoil fold, however, differently from canonical Kunitz type (STI) inhibitors, its reactive site loop has an insertion of one residue, Glu68, between the residues P1 and P2. Surprisingly, DrTI is an effective inhibitor of trypsin and human plasma kallikrein, but not of chymotrypsin and tissue kallikrein. Putative structural grounds of such specificity are discussed. (C) 2003 Elsevier Inc. All rights reserved.