Plasma fibronectin was determined in 121 normal adults and in 149 patients. Fibronectin levels in normals were strongly influenced by sex and age. The mean value of the protein in cancer patients did not differ from that in normal controls; however, patients with cryofibrinogenaemia or extensive liver metastases had lower values whereas those with obstructive jaundice due to pancreatic carcinoma had higher values than normal controls. Fibronectin levels were greatly increased in patients with primary biliary cirrhosis and moderately elevated in nephrotic syndrome. In patients with severe infection or sepsis, plasma fibronectin did not show a consistent pattern. Patients with overt disseminated intravascular coagulation, irrespective of its cause, had the lowest plasma fibronectin concentrations.