Ascitic fluid concentrations of fibronectin, cholesterol and protein were determined in 95 patients: 38 with cirrhosis of the liver, 10 with miscellaneous nonmalignant diseases, 43 with peritoneal carcinomatosis and 4 with liver metastases or hepatocellular carcinoma. Fibronectin, cholesterol and protein at discrimination values of 7.5 mg/100 ml, 45 mg/100 ml and 3.0 g/100 ml, respectively, separated patients with peritoneal carcinomatosis from patients with cirrhosis with an efficiency of 94%, 90% and 85%, respectively. Thus, ascitic fluid determinations of fibronectin and cholesterol offer good discrimination of cirrhotic ascites from ascites related to peritoneal carcinomatosis, superior to the conventional protein determination. However, the failure of all parameters to distinguish ascites caused by miscellaneous nonmalignant diseases from malignancy-related ascites underscores the importance of highly specific methods to confirm a suspected diagnosis of malignancy-related ascites.