A quasi-planar X-ray study has been done to assess the quality of crystals of pure thaumatin and of hen egg-white lysozyme as well as of lysozyme which was intentionally contaminated by structurally unrelated (ovalbumin and conalbumin) or related (turkey egg-white lysozyme) macromolecules. To investigate the behavior of different growth sectors, we have chosen crystals exhibiting well defined habit, namely crystals grown either in agarose gel or in silica gel. The main defect evidenced is a misorientation originating at the level of the nucleus: the parts grown in the + c and - c directions seem to be individuals twisted always clockwise with respect to the growth direction. The measured lattice plane tilt increases up to 3.5 min of arc when the contaminant content increases. In addition to this defect, we could measure for lysozyme relative lattice parameters differences (Deltad/d) of the order of 10(-4) between prismatic and pyramidal growth sectors. All these defects do not seem to have a major influence on the resolution limit but they have consequences on optical properties.