1. Sephadex in bead form shows reversible changes of inner volume when immersed in solutions of a non-penetrating solute. These changes are in accordance with the theory of Flory (1953) for the swelling of gels. This makes possible the use of single beads for measuring the osmotic pressures of polymer solutions, up to or beyond 1kg./cm.2. 2. Measurements of the inner volume of Sephadex G-200 by equilibrium dilution, at various concentrations of dextran 500, gave values in agreement with those obtained from the dimensions of single beads. 3. Measurements by chromatography of the inner volume of Sephadex G-200 gave values that differed slightly but significantly from those obtained by the other methods. Similar small disagreements were found between the dilution and chromatographic values for bovine serum albumin measured in the presence of various concentrations of dextran 500. The major factor that affects the dependence of the chromatographic elution volume (of dextran 500) on concentration is shown to be the change of the inner volume of the Sephadex, with dynamic factors playing a minor role. 4. Contrary to the findings of Ackers (1964), the equilibrium dilution and chromatographic methods gave closely agreeing values for the distribution coefficient, Kav., of bovine serum albumin on Sephadex G-200; this cast doubt on his explanation of the nature of chromatographic separations on Sephadex.
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