The absolute configuration of haem A, the prosthetic group of cytochrome c oxidase, was determined to be S by analysis of the bond angles surrounding the chiral centre of haem A after refinement with X-PLOR starting from respective initial structures with R and S configurations under absolute configuration constraints at 1.8 Angstrom resolution. The same result was obtained by refinement at 1.8 Angstrom resolution without the absolute configuration constraints. Both of these methods were applicable down to a resolution of about 2.8 Angstrom. The constrained refinement converges more quickly than the unconstrained refinement.