Insoluble complexes, formed by electrostatic interaction between chondromucoprotein and chromoproteins (haemoglobin, methaemalbumin), were studied by measurement of precipitated pigment and by decrease in peroxidatic activity, maximum formation from aqueous solution occurring at pH 4.0-4.6. 2. Chondromucoprotein did not form complexes with plasma haptoglobins and haptohaemoglobins under these conditions, and high concentrations had no significant effect on colorimetric estimates of serum haptoglobin, although the peroxidatic activity of haemoglobinaemic serum was depressed owing to formation of chondromucoprotein-methaemalbumin complex. 3. The complexes formed by interaction between chondromucoprotein and plasma proteins contain two protein-bound biologically active components (plasminogen, haematin), as a result of co-precipitation after interaction between their carriers and chondromucoprotein. The possible presence of other biologically active trace components is discussed. 4. The results are related to complex-formation between other plasma proteins and chondromucoprotein, and possible implications arising from the complex-forming properties of tissue and urine chondromucoprotein are referred to. It is concluded that the inability of chondromucoprotein to form complexes with normal urine proteins is due to a deficiency of fibrinogen, beta-lipoproteins and chromoproteins, which, in plasma, form a large proportion of the proteins involved in complex-formation.
