Trypanothione reductase from Trypanosoma cruzi is the most promising target molecule for the rational design of a specific drug against Chagas' disease. The recombinant protein was purified in a single chromatographic step and crystallized. Two crystal forms suitable for X-ray diffraction analysis were obtained. Tetragonal crystals (a = b = 87.4 A, c = 152.3 A) were grown from 30% polyethylene glycol (average M(r) = 8,000) in the presence of 0.2% beta-n-octylglucoside (space group either P4(2) with one dimer or P4(2)22 with one monomer in the asymmetric unit). Monoclinic crystals (space group P2, a = 136.3 A, b = 91.1 A, c = 126.0 A, beta = 94 degrees) were grown from 1.2 M sodium citrate in the presence of 2% octanoyl-N-methyl-glucamide. They contain two dimers of the enzyme in the asymmetric unit; both crystal forms diffract to 3 A resolution.