A high-Mr subunit was prepared from durum wheat (Triticum durum). Viscometric analysis showed that the molecule is rod-shaped, with molecular dimensions of about 50 nm x 1.75 nm (500 A x 17.5 A) in 0.05 M-acetic acid/0.01 M-glycine and 49 nm x 1.79 nm (490 A x 17.9 A) in aq. 50% (v/v) propan-1-ol (+/- 0.01 M-glycine) at 30 degrees C. C.d. spectroscopy in the same solvents indicated the presence of beta-turns, but little alpha-helix [7% in 50% (v/v) propan-1-ol] and no beta-sheet. However, when dissolved in trifluoroethanol the protein contains about 30% alpha-helix, and viscometric analysis gives dimensions of about 62 nm x 1.53 nm (620 A x 15.3 A). It is proposed, on the basis of these studies and previously published structural prediction, that the repetitive central domain of the high-Mr subunit forms a loose spiral based on repetitive beta-turns, whereas the shorter non-repetitive N- and C-terminal domains are alpha-helical in trifluoroethanol, but random coil in other solvents. The Mr of the high-Mr subunit determined from the intrinsic viscosity in 6.0 M-guanidinium chloride was 65,000, compared with 84,000 determined in 5.0 M-guanidinium thiocyanate. The latter value is consistent with the Mr values for related proteins whose complete amino acid sequences are known, and it was concluded that the protein is incompletely denatured in the former solvent. This was confirmed by c.d. spectroscopy in increasing concentrations (1-6 M) of guanidinium chloride.