We sought to develop a rodent model of spontaneous bacterial peritonitis and report here the preliminary results of carbon tetrachloride-induced cirrhosis in which ascites and bacterial peritonitis predictably develop. Of 41 rats that survived the initial carbon tetrachloride toxicity, 38 (92.7%) developed cirrhosis with ascites. Of these 38, 21 (55.3%) developed 24 episodes of ascitic fluid infection without iatrogenic colonization. No surgically treatable source of infection was identified at autopsy in any rat; therefore, the infections were presumed to be "spontaneous." Eight (50%) of the 16 rats with culture-positive ascitic fluid at postmortem examination also had spontaneous pleural fluid infection with the same organism. Escherichia coli and Proteus sp. were the organisms most commonly isolated. This rodent model of cirrhosis with ascites appears to be the first high-yield animal model of spontaneous bacterial peritonitis. Ascitic fluid infection in these rats resembles ascitic fluid infection in humans. This model will allow further investigation of the mechanisms of pathogenesis of ascitic fluid infection and provide insight into the prevention and treatment of spontaneous bacterial peritonitis and pleural fluid infection in patients with cirrhosis.