In this prospective study, we tried to evaluate various "humoral tests of malignancy" regarding their efficiency of discriminating between malignancy-related and non-malignant ascites. Fibronectin, total protein, number of cells, LDH, pH, specific gravity and cytology were compared in the ascitic fluid of 51 patients with malignancy-related and 52 patients with non-malignant ascites; patients with tuberculous peritonitis were not included. Ascitic fluid cholesterol was determined in 36 of 51 malignancy-related and in 37 of 52 non-malignant ascites. Cytology and fibronectin were found 100% specific with diagnostic efficiency 87.5% and 94.2% respectively under optimal conditions. Cholesterol was neither sensitive nor specific. It is concluded that fibronectin was a valuable test for malignancy-associated ascites.