Binding of human blood plasma proteins with hyaluronic acid, chondroitin-4-SO4 and heparin was studied. Chondroitin-4-SO4 was not involved in the reaction, whereas heparin and hyaluronic acid formed complexes with the proteins at very low concentrations. The intensity of binding correlated with concentration of carbohydrates in blood plasma proteins. Heparin displaced hyaluronic acid in the complexes. Biological importance of the interactions between glycosaminoglycans and glycoproteins, incorporated into blood plasma as well as into cell membranes of various tissues, is discussed.