1. Sedimentation-velocity, intrinsic-viscosity and partial-specific-volume measurements on a typical blood-group-specific glycoprotein are reported for a range of environmental conditions. 2. The sedimentation coefficients, S, are strongly concentration-dependent, and follow the reciprocal law; the limiting values at 2°, 25° and 45°, after correction to 25°, show slight dependence on temperature. 3. The intrinsic viscosities, [η], at 25° and 45° show more marked temperature-dependence, and correspond to a very asymmetric or very expanded molecular conformation. 4. From the value of the ratio K/[η], where K=S0.d(1/S)/dc, it is concluded that the molecular conformation is roughly spherical; application of the Einstein viscosity equation then suggests an expansion factor of about 60, compatible with a flexible configuration approaching that of a random coil. 5. The sedimentation coefficient is not affected by variation of ionic strength in the range 0·01–0·50, nor by pH in the range 3–10. 6. Sodium dodecyl sulphate at 1·5% produces a small decrease in S; the effect is greater than would be expected from the observed extent of binding, but is too small to correspond to a significant change in secondary structure; the serological activity is unaffected by sodium dodecyl sulphate. 7. All the properties observed indicate the absence of any secondary structure in blood-group substances.

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