AaH-IV, a P-III-type metalloproteinase found in Agkistrodon acutus venom, readily cleaves itself to release a stable protein named acucetin at 310 K under neutral and weakly alkaline conditions. A partial amino-acid residue sequence of acucetin indicates that the protein has a high homology to snake-venom proteins containing both disintegrin-like and cysteine-rich domains. Acucetin has been crystallized in space group R32, with hexagonal unit-cell parameters a = b = 155.98, c = 76.07 A. The V(M) value of about 2.97 A(3) Da(-1) suggests the presence of only one molecule in the asymmetric unit.