A computer program, called OOPS, is described which facilitates and speeds up the process of rebuilding a protein structure inside its electron density and reduces the chances of local errors persevering throughout the crystallographic protein structure determination process. The program uses a set of criteria to judge how reasonable each protein residue is and it generates macros for the macromolecular crystallographic model-building program O [Jones, Zou, Cowan & Kjeldgaard (1991). Acta Cryst. A47, 110-119] which, when executed, will take the crystallographer on a journey along all suspect residues.