The enzyme alliinase has been isolated from garlic bulbs and crystallized. The crystals belong to space group P2(1), with unit-cell parameters a = 70.191, b = 127.006, c = 108.085 A, beta = 93.384 degrees. They diffract to 2.2 A at liquid-nitrogen temperature. Analysis of the Patterson self-rotation function suggests that the crystals contain two dimeric molecules per asymmetric unit.