A trypsin inhibitor from Opuntia streptacantha Lemaire (Prickly pear) seeds was purified and characterized. Of several proteases tested, this inhibitor showed specificity to trypsin-like enzymes. The major inhibitor present in these seeds showed distinctive characteristics, most notably a low molecular weight of 4.19 kDa, as determined by MALDI TOF, and an unusually high thermal stability, retaining most of the activity after heating the sample 1h to 120 degrees C with a pressure of 1 kg/cm(2). Its complete amino acid sequence was obtained through mass spectrometry, this establishing presence a blocked N-terminal region. When comparing its sequence in the MEROPS database for peptidases and peptidase inhibitors, it showed 34.48% identity with a serine-proteinase inhibitor from the I15 family.