Plants constitute an important source of compounds which can induce apoptosis in a variety of cells. Previously, we reported the isolation of a trypsin inhibitor from Peltophorum dubium seeds (PDTI). This inhibitor, as well as soybean trypsin inhibitor (SBTI), both belonging to the Kunitz family, have lectin-like properties and trigger rat lymphoma cell apoptosis. In the present study, we demonstrate for the first time that PDTI and SBTI induce human leukemia Jurkat cell death. Induction of apoptosis was confirmed by flow cytometry after propidium iodide labeling of apoptotic nuclei, showing a considerable increase of the sub G(0)/G(1) fraction, with no cell cycle arrest. With the purpose of gaining insight into the signaling pathways involved, we investigated the activation of caspases and the effect of caspase inhibitors, and showed caspases-3 and -8-like activation by PDTI or SBTI-treatment. Consistent with these results, pan caspase inhibitor and caspase-8 inhibitor protected Jurkat cells from apoptosis. However, there was no caspase-9 activation, confirmed by the failure of caspase-9 inhibitor to prevent cell death. No significant release of cytochrome c from mitochondria was detected suggesting that the intrinsic mitochondrial pathway is not predominant in the apoptotic process. On the other hand, recruitment of Fas-associated death domain (FADD) to the cell membrane indicates the involvement of this adaptor protein in PDTI- and SBTI-induced apoptosis in Jurkat cells. Furthermore, human peripheral lymphocytes, either stimulated with phytohemagglutinin or not, are also susceptible to viability decrease induced by these inhibitors.