Cys49Ser mutant Helicobacter pylori alkyl hydroperoxide reductase (C49S HpAhpC) was purified under reducing conditions in monomeric and decameric forms. The monomeric form was crystallized by the hanging-drop vapour-diffusion method. The crystals diffracted to 2.25 A resolution and belonged to space group C2, with unit-cell parameters a = 245.8, b = 140.7, c = 189.5 A, beta = 127 degrees , and contained 20 molecules in the asymmetric unit. A crystal of the decameric form was obtained by the microbatch crystallization method and diffracted to 2.8 A resolution. It belonged to space group C222, with unit-cell parameters a = 257.5, b = 417.5, c = 95.6 A. The structure of the monomeric form of C49S HpAhpC has been solved by the molecular-replacement method.