Selective intestinal decontamination for 7 days with norfloxacin was performed in 14 cirrhotic patients with ascites and low ascitic fluid total protein. Variations in serum and ascitic fluid of C3 and C4 and ascitic fluid total protein after therapy were compared with those of a control group of 14 untreated patients with similar characteristics. After oral norfloxacin administration, we saw a significant increase of C3 in serum (p < 0.05) and ascitic fluid (p = 0.01). A significant increase was also observed in ascitic fluid total protein (p < 0.05) but not in serum and ascitic fluid C4. There were no changes in serum C3, ascitic fluid C3, ascitic fluid C4 or in ascitic fluid total protein in group 2. These data demonstrate that selective intestinal decontamination increases serum and ascitic fluid C3 levels and, therefore, might be useful in preventing spontaneous infections in cirrhotic patients at high risk of infection. (HEPATOLOGY 1990;12:1175–1178).