A snake-venom protein named AHP-LAAO has been purified from Agkistrodon halys pallas venom using four-stage chromatography. AHP-LAAO is a novel member of the snake-venom L-amino-acid oxidase family. Its amino-acid sequence shows high homology to other members of this family. For L-leucine, the values of k cat and K M are 31.1 s -1 and 0.25 mM, respectively. The molecular weight of AHP-LAAO is about 60.7 kDa as determined by MALDI-TOF mass spectrometry. AHP-LAAO can also induce apoptosis of cultured Hela cells. Two sets of diffraction data with similar resolution limits (about 2.5 Å) were collected independently at MacCHESS (Cornell High Energy Synchrotron Source, USA) and IHEP (Institute of High Energy Physics, Beijing, China). The crystals belong to space group I2 13, with unit-cell parameter a = 169.31 Å, corresponding to one molecule in the asymmetric unit and a volume-to-weight ratio of 3.33 Å 3 Da -1. The final structural model is similar to that of L-amino-acid oxidase from Calloselasma rhodostoma venom. © 2004 International Union of Crystallography. Printed in Denmark - all rights reserved.