Swelling pressures of cartilage slices from human femoral heads and condyles have been measured against polyethylene glycol 20,000 solutions, standardised previously against mechanically applied pressure. In order to express the fixed charge density of cartilage on the basis of extrafibrillar water, the fraction of intrafibrillar water was determined from the partition of serum albumin between cartilage depleted of proteoglycans, and outside saline solution. The value for the intrafibrillar water was found to be 0.7 x collagen content. Provided the fixed charge density of cartilage slices is based on the equilibrium extrafibrillar water, the swelling pressure curve for cartilage coincides with that for isolated proteoglycans, except at or near 100% hydration when the tensile stresses present in the collagen network counteract the osmotic pressure of the proteoglycans. These tensile stresses are relieved as soon as the tissue is compressed. The maintenance of the tissue hydration under load can thus be completely described in terms of a balance between the osmotic pressure of the proteoglycans and the applied pressure.