Flours of wheat, rye, barley, oats, maize and sorghum have been extracted to remove albumins, globulins and prolamins. The SS bonds of the starch-glutelin residues (4 to 8 % protein) have been reduced with sulphite and titrated with phenyl mercury acetate (PMA) in the presence of 3 M-guanidine hydrochloride at 37 "C and, in its absence, at 2 and 37 "C. Attention has been paid to several possible sources of error including oxidation and the rate of diffusion of PMA into protein particles. Approximate values for the diffusion coefficients of PMA through cellulose and protein films were obtained. Titrations at 37 "C in the absence of guanidine hydrochloride are unsatisfactory due to reaction of intrachain SS links. Evidence in the literature suggests that the SS bonds titratable at 2 "Care inter-chain, possibly including somestrained intra-chain bonds. The results imply that most of the major polypeptide chains in the cereal glutelins examined, apart from barley and sorghum, contain two such bonds. In the case of barley glutelin probably less than half the chains have two labile SS bonds. Most of the chains in sorghum glutelin appear to have a single labile bond and the polymers may contain only a few chains. The molecular weights (in thousands) of the principal polypeptide chains of the glutelins, deduced from gel electrophoresis in sodium dodecyl sulphate (SDS), are: Cappelle-Desprez wheat 44, 41 ; Manitoba wheat, rye and barley, 44; oats, 3 3 , 2 3 ; maize, 23, 19; sorghum, 22, 18).