In order to determine the composition of "normal" ascitic fluid, the results of analysis of the first paracentesis on 347 consecutive cirrhotic patients with ascites at the West Haven Veterans Administration Hospital between 1955 and 1976 were examined. The ascites was considered "normal" in 259 patients. Bacterial peritonitis was present in 51, malignant ascites in 18, pancreatitic ascites in 15, and ascites of other types in 4 patients. Normal ascites is sterile, usually clear, and contains 281 +/- 25 leukocytes/mm3 (mean +/- SEM), 27 +/- 2% of which are polymorphonuclear. In spontaneous bacterial peritonitis the fluid is usually cloudy, contains 6084 +/- 858 white blood cells/mm3, 77 +/- 4% of which were PMN and culture is positive for a single bacterial species, usually enteric in origin. Malignant and pancreatitis ascites are sterile, often cloudy, and contain an average of 696 +/- 273 and 1821 +/- 833 leukocytes/mm3, respectively, about half of which are polymorphonuclear. Amylase activity is increased in pancreatitic ascites, but not in other types of ascites. Stained smears of sediment for bacteria are often positive in bacterial peritonitis, but not in the other categories. Neither the specific gravity, protein concentration, nor glucose level is useful in the differential diagnosis of ascites. Based on the critical number of leukocytes alone, (500/mm3), one can accurately differentiate infected from uninfected fluid in over 90% of ascitic patients.