[en] Protein disulfide isomerase (PDI) is known to bind to the thyroid hormone triiodothyronine (T3). Here, the expression, crystallization and preliminary X-ray studies of the b and b′ domains of rat PDI in ligand-free and T3-complexed states are reported. Protein disulfide isomerase (PDI) is a multifunctional protein that catalyzes the formation of a disulfide bond in nascent and misfolded proteins and is also known to bind to the thyroid hormone triiodothyronine (T3). When T3 is bound to PDI its catalytic activity is inhibited, but the biological function of this binding is not well understood. In previous studies, it was found that T3 binds to the bb′ fragment of PDI. Therefore, to clarify the structure of the complex consisting of PDI bound to T3, a crystallographic analysis of the three-dimensional structure of the T3–rat PDI bb′ complex was performed. Native bb′ crystals and T3–bb′ complex crystals were both obtained using the hanging-drop vapour-diffusion technique with 1.6 M trisodium citrate pH 6.2 as a precipitant. The space group of the native bb′ crystals was found to be C222, with unit-cell parameters a = 94.8, b = 114.9, c = 182.9 Å, while the space group of the T3–bb′ complex crystals was P2₁2₁2₁, with unit-cell parameters a = 99.9, b = 184.5, c = 232.2 Å. Diffraction data for the native and complex crystals were collected to resolutions of 3.06 and 3.00 Å, respectively