Crystals suitable for an X-ray diffraction investigation have been obtained of recombinant Coprinus cinereus peroxidase expressed in Aspergillus oryzae. The crystals were grown by the hanging drop method with polyethylene glycol 6000 as the precipitant. A pH range from 6.2 to 8.0 and CaCl2 or MgCl2 present at a concentration of 0.35 M were essential for the crystal growth. A metastable monoclinic modification can be obtained under certain conditions, and with variations in temperature they are transformed into a stable orthorhombic modification. With CaCl2 as the additive, the unit cell dimensions were a = 74.9 A, b = 76.8 A and c = 128.2 A. With two peroxidase molecules per asymmetric unit, the solvent content is 49% (v/v). In the diffraction pattern, the reflections Okl are systematically very weak for k = 2n + 1. Combined with an analysis of the Patterson function, this showed that the two independent molecules are related by the pseudotranslational symmetry 0.29a + 0.5b. The possible space groups are P2(1)2(1)2(1) or P2(1)22(1) because of this pseudosymmetry. The crystals diffract to a resolution of 2.9 A.