Ascitic fluid infection probably results from repeated episodes of bacteremia and seeding of ascitic fluid. The outcome of these episodes of colonization is probably a function of serum and ascitic fluid defense mechanisms and the virulence of the organism. Patients who develop spontaneous bacterial peritonitis may have serum and ascitic fluid characteristics that are different from those who do not develop infection. We prospectively collected serum and ascitic fluid specimens at the time of admission from patients with sterile cirrhotic ascites, and tested these specimens for interleukin-6, tumor necrosis factor-alpha, and nitric oxide and compared these results as well as other characteristics of patients who did not develop infection to those who did. An elevated baseline serum tumor necrosis factor-alpha as well as an increased proportion of polymorphonuclear leukocytes in sterile ascitic fluid from patients who subsequently developed infection probably represent a subclinical activation of defense mechanisms from prior silent colonizations with bacteria.