Serial ascitic fluid samples were obtained during diuresis in seven patients with portal hypertension-related ascites. The samples were tested for concentrations of total protein, CH100, C3 and C4 as well as for in vitro opsonic activity. These parameters were all found to increase to a statistically significant degree when the initial specimen was compared to the final specimen: total protein = 1.5 vs. 2.7 gm per dl; CH100 = 9.3 vs. 20.2 units per ml; C3 = 13.4 vs. 23.8 mg per dl; C4 = 1.9 vs. 3.6 mg per dl, and opsonic activity = 0.8 vs. 1.9 log kill. This increased opsonic activity resulted in a greater than 10-fold increase in bacterial killing. This study demonstrates that diuresis of patients with cirrhotic ascites increases the concentrations of ascitic fluid complement components and increases the opsonic activity of ascitic fluid and may help protect patients from bacterial infection of their ascites.