The toxin vipoxin, which is a complex between a basic toxic phospholipase A2 and an acidic non-toxic protein inhibitor, is found in the venom of the Bulgarian viper (Vipera ammodytes ammodytes), the most toxic snake in Europe. The two polypeptide chains each consist of 122 residues and are highly homologous (62%). The vipoxin complex is the first reported example of a high degree of structural homology between an enzyme and its natural inhibitor. The present crystals diffract in the X-ray beam to 1.8 A resolution. The space group is P2(1)2(1)2(1). The cell dimensions are a = 45.80 A, b = 55.36 A and c = 107.69 A. Native data to a resolution of 2.8 A have been recorded.