Two new trypsin inhibitors, TDI-I and TDI-II, were purified from the seeds of the native Brazilian tree Copaifera langsdourfii (Caesalpinoideae, Leguminosae). The purification procedure involved ammonium sulfate fractionation, ion-exchange chromatography on DEAE-Sepharose, affinity chromatography on trypsin-Sepharose, and reversed-phase (RP) HPLC. RP-HPLC yielded two forms (TDI-I and TDI-II), as confirmed by isoelectric focusing, with pi values between 7.0 and 8.1. The molecular mass of the TDI forms was 24 kDa based on FPLC gel filtration on Superdex 75. Under reducing conditions in tricine SDS-PAGE the molecular masses of TDI-I and TDI-II were 12 and 10 kDa, respectively. The k(i) values were 1.1 and 1.2 nM for TDI-I and TDI-II, respectively, and there was no inhibitory effect on chymotrypsin. Amino acid analysis revealed high levels of aspartic acid, glutamic acid, serine, glycine, proline, and lysine but low levels of methionine and aromatic amino acids in both inhibitors; the calculated molecular masses were 11,456 and 10,008 for TDI-I and II, respectively. Eased on the N-terminal sequences of TDI-I and TDI-II, TDI-I belongs to the Kunitz family of trypsin inhibitors, whereas TDI-II showed no homology to any other protein. This observation suggests that TDI-II belongs to a new inhibitor subclass of low-molecular mass proteins in the subfamily Caesalpinoideae.20117