Crystallization and preliminary X-ray diffraction studies of Gcy1p, an aldo-keto reductase from Saccharomyces cerevisiae, have been performed. Both the wild type and a double-mutant form of Gcy1p were crystallized using the hanging-drop method at 298 K; however, only the double-mutant form has so far yielded crystals suitable for X-ray diffraction analysis. These crystals belonged to the primitive monoclinic space group P2(1), with unit-cell parameters a = 50.94, b = 65.64, c = 86.23 A, beta = 92.64 degrees. Diffraction data were collected to 2.5 A. Assuming two 35 kDa subunits in the asymmetric unit yielded a V(m) of 2.06 A(3) Da(-1). Additionally, a kinetic study performed by measuring the rate of oxidation of NADPH in the presence of several substrates indicates that both wild-type and double-mutant proteins are enzymes possessing NADPH-dependent reductase activity.