Phospholipase A2 inhibitor, the non-toxic, acidic component of vipoxin from the venom of Bulgarian viper (Vipera ammodytes), has been crystallized. The tetragonal crystals obtained, exhibit the symmetry of space group P4(1)22 (or 4(3)22) with unit cell dimensions a = b = 59.9 A; c = 141.1 A; alpha = beta = gamma = 90 degrees. For two molecules per asymmetric unit, this would give Vm = 2.3 A3/Da, indicating normal packing. The crystals diffract to 2.5 A and a native data set to 3.2 A resolution has been collected.