1. Observations are reported on the variation of the swelling of sulphoethyl-Sephadex C-50 and C-25, and of the partition of NaCl between solution and gel, with the concentration of NaCl. 2. The results were interpreted in terms of Manning's (1969) treatment of the interactions between polyionic polymers and simple electrolytes and of Flory's (1953) treatment of the swelling of gels. 3. The results indicate net inner osmotic pressures as high as 3.6×10 5 Pa (140 mosmolar=3.6×10 5 N/m 2 ) in 0.15 m -NaCl. 4. It is suggested that cartilage may have net inner osmotic pressure of the order of 10 5 Pa.

1. The measurement of osmotic pressure by means of a single bead of Sephadex (Edmond, Farquhar, Dunstone & Ogston, 1968) has been made more precise by immobilizing the bead on a fine needle. The design, calibration and use of the osmometer are described. 2. The method is particularly suitable for measuring high osmotic pressures in solutions of high-molecular-weight solutes, which must not penetrate the Sephadex to a significant extent. 3. With Sephadex G-50 the limit of precision is about 1.5cmH 2 O and the lower limit of molecular weight for a solute of compact molecular form is about 10 5 . 4. The time required for each equilibration is less than 10min. 5. An impaled bead can be stored in the dry state without affecting its calibration. 6. Measurements on a sample of polyvinyl alcohol, degree of polymerization stated as 1750±50, gave ¯m n 43000±3000 and A 2 (6.0±0.24)×10 −4 .