Patients with liver cirrhosis with ascites and low levels of C3 and Total Proteins in ascitic fluid show a greater predisposition to the development of spontaneous bacterial peritonitis. The variation of C3 and Total Proteins levels in ascitic fluid in two groups of patients with liver cirrhosis was studied. Group I (n = 14) underwent intestinal sterilization (n = 7) or selective intestinal decontamination (= 7). Group II was a control. A statistically significant increase in C3 (p less than 0.01) and Total Proteins (p less than 0.015) levels in ascitic fluid in Group I patients was found. There were no changes in the control group. The incidence of side effects was higher in those patients who underwent intestinal sterilization. Our results suggest that selective intestinal decontamination may be useful as a prophylactic measure against spontaneous bacterial peritonitis in those patients with liver cirrhosis at high risk of infection by increasing the bactericidal capacity of ascitic fluid.