Aldehyde reductase from porcine kidney has been crystallized from buffered ammonium sulfate solutions. Two crystal forms are monoclinic, space group P2(1), with a = 56.2, b = 98.1, c = 73.2 A, beta = 112.5 degrees and a = 92.4, b = 62.1, c = 59.0 A, beta = 94.6 degrees. A third crystal form is hexagonal with a = b = 166.0, c = 66.0 A, alpha = beta = 90.0 degrees and gamma = 120.0 degrees. Molecular-replacement structure solutions have been successfully obtained for the two monoclinic crystal forms. The crystallographic R factor at 8-2.8 A resolution for the two monoclinic crystal forms is currently 0.23 and 0.25, respectively. There are two molecules per asymmetric unit related by a non-crystallographic twofold axis. The aldehyde reductase models are supported by the arrangement of the molecules in their respective unit cells and by electron densities corresponding to amino-acid side chains not included in the search structures.