Some preliminary X-ray observations on crystalline monoclinic ribonuclease have already been published (Carlisle & Scouloudi 1951), but we should like to recall, very briefly, some of the points made in this early work. The air-dried crystal gave a molecular weight of about 13400 for this enzyme. Two somewhat elongated molecules, each roughly 30 x 18 x 48 Å, appeared to fit into the wet unit cell whose dimensions were a = 30⋅90 Å, b = 38⋅8 Å, c = 54⋅06 Å, β = 106°, with the space group P 2 1 . The c -axis Patterson projection of the wet crystal showed an hexagonal distribution of peaks situated at about 9⋅5 Å from one another. Using the arguments of Perutz (1949), it was suggested that the polypeptide chains of ribonuclease, packed in hexagonal array, should be lying parallel to the c -axis of the crystal. Some confirmation of this possibility was obtained from the a - and b -axis Patterson projection maps of the wet crystals, which showed peaks lying at intervals of about 5⋅0 Å approximately parallel to the c -axis. We concluded, very tentatively, that the molecule should contain at least five ‘crystallographic’ polypeptide chains and that there should be about two amino-acids per repeat of the ∝ -fold. We have since carried out a three-dimensional Patterson analysis of this crystal, using some 5000 reflexions, for values of h up to 20, k to 16, and l to 30, and the section at V = 0 showed a repeat of peaks lying at about 5⋅4 Å from another along the c -axis. This seemed to confirm our view that the polypeptide chains of this molecule were lying nearly parallel to the c -axis of the crystal.