1. The use of gel osmometry for osmotic pressure measurements on polyelectrolyte solutions is discussed, with special reference to the effect of interaction of low molecular weight supporting electrolyte with the gel, and to the effect of penetration of the gel phase by a small multivalent fraction of a polydisperse polyelectrolyte. 2. Measurements are reported of sodium chloride activity and of osmotic pressure for the system sodium dextran sulphate+sodium chloride+water. 3. The results agree well, over a wide range of concentration, with an extended version (Wells 1973) of the Manning (1969) theory of polyelectrolyte solutions. 4. Dextran sulphate is considered to be a suitable model for connective tissue proteoglycan. The colloid osmotic pressures for proteoglycan solutions under physiological conditions are calculated in order to obtain an estimate of the internal osmotic pressure of cartilage. The effects of variations in the composition of the proteoglycan, and hence of its charge density, are examined.