Acid glycosaminoglycans were isolated from normal human plasma: (a) following fractionation of plasma (with protease inhibitors) on Sephadex G-200; (b) by ultrafiltration through membranes with retention of molecules above 50 kDa, with and without previous addition of NaCl, Triton X-100, urea, or guanidine HCl; (c) by filtering on Ecteola-cellulose either untreated plasma or after treatment with NaCl, urea, Triton X-100, papain or NaOH. More than 95% of plasma glycosaminoglycans interact with plasma proteins to give complexes that exhibit reproducible behaviour on Sephadex G-200 and are retained by ultrafiltration membranes, which the 12-20 kDa polysaccharide chains do filter. High charge plasma glycosaminoglycans show ionic interactions with proteins, while low charge glycosaminoglycan interactions are resistant to Ecteola charged groups, to 0.5% Triton and 4 M urea, while not to 4 M guanidine HC1. Some glycosaminoglycan-protein complexes appear resistant to proteolysis, suggesting that they may originate from lymphocytes. The simple method utilized for plasma GAG measurement may represent an useful tool in clinical practice.