An acidic phospholipase A(2) was purified from Deinagkistrodon acutus (Agkistrodon acutus) which displays an inhibitory effect on platelet aggregation. The three-dimensional structure of the enzyme was determined by molecular replacement at 2.6 A resolution with a crystallographic R factor of 18.40% (R(free) = 22.50%) and reasonable stereochemistry. Two molecules in the asymmetric unit form a dimer and the dimer formation accompanies a significant conformational adaptation of segment 14-23, a constituent of the 'interface recognition site' (IRS). This probably reflects the inherent structural flexibility of the IRS. The possible expansion of the site for inhibiting platelet aggregation as proposed previously [Wang et al. (1996), J. Mol. Biol. 255, 669-676] is discussed.