The enzyme catalase (H(2)O(2)-H(2)O(2) oxidoredutase; E.C. 11.1.6) was purified from haemolysate of human placenta and crystallized using the vapour-diffusion technique. Synchrotron-radiation diffraction data have been collected to 1.76 A resolution. The enzyme crystallized in the space group P2(1)2(1)2(1), with unit-cell dimensions a = 83.6, b = 139.4, c = 227.5 A. A molecular-replacement solution of the structure has been obtained using beef liver catalase (PDB code 4blc) as a search model.