L-Threonine dehydrogenase from Pyrococcus horikoshii (TDH) is a water-soluble metalloenzyme, the molecular structure of which has been unknown until recently. The Zn2+ ion in the native TDH, prepared as a recombinant protein, is replaced artificially with Co2+, Ni2+ or Gd3+. These samples are crystallized in homogeneous magnetic fields of 2-10T. Half of the Co- or Gd-substituted crystals show magnetic orientation in a field of 2 T at 278 K whereas the crystals of the native TDH require a 4 T magnetic field for half orientation. The sensitivity to magnetic orientation can thus be increased by metal substitution. On the other hand, we cannot assign clear changes in the size, number, and quality of the native and metal-substituted crystals with and without the presence of the magnetic field. (C) 2009 Elsevier B.V. All rights reserved.