We have analyzed the crystal quality along a capillary by a precise protocol that comprises the study of tetragonal lysozyme cylindrical crystals that fill the capillary diameter (i.e. rods), the careful definition of the diffraction parameters and the use of a single software for the data reduction in order to avoid any bias in the comparison of the quality of different data sets. Our results cannot be explained on the basis of the different redundancy of the data sets and they demonstrate that the gel acupuncture method promotes a gradient of supersaturation along the capillary that yields in the same experiment crystals of increasing quality as a function of the position. However, despite being single crystals, rods have regions that show different crystal quality because they grew at different supersaturations. Our data are in agreement with the existence of a relation between length of the c-axis and crystal quality reported by other groups, but a deeper analysis of the cell parameters reveals the existence of a significant linear relation (R = 0.87) with the c/a-axis ratio. This result points to the hypothesis of an ideal unit cell that yields the best crystals in terms of I/sigma(I). (C) 2002 Elsevier Science B.V. All rights reserved.