1. Although glycoproteins with less than 1% of sialic acid (fibrinogen, lipoproteins, gamma-globulins) interact electrostatically with chondromucoprotein to form insoluble complexes, interaction with glycoproteins containing larger amounts of sialic acid (orosomucoid, urine glycoprotein, seromucoid, fraction VI) was electrostatically impossible. Reasons for this are discussed. 2. The latter glycoproteins interacted with chondromucoprotein after mild acid hydrolysis or neuraminidase treatment, complex-formation being inversely related to their sialic acid content. 3. Complex-formation with sialic acid-deficient orosomucoid was maximum at pH3.6 and negligible above its isoelectric point of pH5, and was inhibited by Ca(2+) ions and EDTA. 4. These results are discussed in relation to the carbohydrate composition and biological activities of euglobulin fractions, and of complexes formed by adding chondromucoprotein to abnormal plasmas which may contain sialic acid-deficient glycoproteins owing to faulty carbohydrate metabolism.