The thermodynamics of interaction between Sephadex and penetrating solutes.

1. We have measured the partition coefficients of bovine serum albumin with Sephadex grades G-100, G-150 and G-200, and of a dextran ([unk](n) 19700) and a polyethylene glycol ([unk](n) 8000) with Sephadex G-200. We have also measured the effects of these solutes on the inner volumes of the grades of Sephadex. 2. The results can be described with fair consistency by means of a simple thermodynamic treatment that makes use of the virial coefficients of Sephadex and of the solute, and of a coefficient that expresses their interaction. This coefficient is related to the ;exclusion volume' of Sephadex for the solutes. 3. The Sephadex G-200-polyethylene glycol system shows anomalies of behaviour that are ascribed to the occurrence of ;incompatible' phase separation within the Sephadex beads.