Cold insoluble globulin (CIG) is a normal glycoprotein of human serum and plasma. The physiological significance of this protein is unknown, but is shows a temperature-dependent relation to fibrinogen and fibrin. It is possible that it represents a substrate for activated fibrin-stabilising factor in the polymerisation of fibrin. CIG is found on the surface of fibroblasts. In the present study CIG was estimated in citrated plasma in 115 patients with rheumatic diseases. Increased amounts were found in patients with systemic lupus erythematosus, secondary amyloidosis in classical and definite rheumatoid arthritis, and in male patients with juvenile rheumatoid arthritis.