A review of the symmetry of proteins has revealed that in many cases the metric symmetry exceeds the reported crystal symmetry. Regardless of the reason, this observation has important implications for experimental protein crystallography. Standard laboratory procedure should always include a direct determination of the lattice metric symmetry. With full knowledge of the highest possible symmetry, the experimentalist is then able to determine in a logical and accurate manner the Laue group and the space group. For those proteins in which it has been proved that the metric symmetry exceeds the crystal symmetry, the protein crystallographer must proceed with caution (e.g.in relating multiple sets of data, positional parametersetc. on the same or related crystals), because a given lattice will have metrically similar unit cells that are not symmetrically equivalent.