A method is proposed for the measurement of glycosaminoglycans (GAG) on 5-10 ml of plasma. It is based on the adsorption of GAG on small ECTEOLA columns followed by measurement of the hexuronic acid in the NaCl eluates. Routine use of the method has indicated the presence of a GAG fraction that adsorbs readily on ECTEOLA ("free" GAG) and of another that adsorbs on it only after treatment with papain ("bound" GAG). "Free" and "bound" GAG have been measured in normal adults, normal children, and children affected by mucopolysaccharidosis type I; the results obtained are in good agreement with those previously reported in the literature.Various analyses performed on purified "free" and "bound" GAG have confirmed that chondroitin-4-sulfate is the main glycosaminoglycan of normal human plasma where it occurs both free and bound to protein and at various levels of sulfation. The presence of small amounts of heparan sulfate and keratan sulfate has also been demonstrated. Metabolic experiments performed in rabbits have indicated that plasma GAG derive from peripheral tissues and increase sharply after papain injection. In young animals the "free" GAG have a faster turnover than the "bound," possibly a reflection of active processes of remodeling and calcification. The synthesis of the "free" and "bound" GAG, as measured with (85)S-sulfate incorporation, seems to proceed at the same rate, and the hypothesis has been advanced that as a result of the action of tissue proteases, part of the "bound" GAG may be transformed into "free" GAG, the latter being immediately extruded from the tissues into the circulatory system.