I have been given the last word in this discussion, and I take advantage of the opportunity with some diffidence. The discussion has ranged widely over the field, but the major issues under dispute have been in the province of the crystallographers. I certainly cannot count myself among them, and am often quite incompetent to judge of the validity of what they say. However, the issues involved are of such importance to protein chemists, and to many biologists as well, that I offer comments on some of them. First of all, it is really amazing, to one who has been in the field of protein chemistry for a quarter of a century, that this discussion could be carried on, at this level, in the year 1952. Even five years ago it would have seemed almost incredible that we could have begun so soon to probe so deeply into the finer details of protein structure. The possibility of thinking in these terms has certainly been profoundly influenced by a development which has not found any but an incidental place in to-day’s discussion—the determination of the sequence of amino-acid residues in polypeptide chains (Sanger 1952). The outstanding achievement in this field hitherto is of course the complete determination of the sequence in the two kinds of peptide chain in insulin, by Sanger & Tuppy (1951) and Sanger & Thompson (1952). It appears that it remains only to fix the nature of the disulphide cross-links in order to have a complete structural formula for at least one protein.