An animal model simulating intra-abdominal sepsis was produced by implanting large bowel contents into the pelvic region of rats. Bacteriological analysis of infected sites showed quantitative differences according to the stage of disease. During the initial, often lethal, peritonitis stage, Escherichia coli (mean concentration, 10(6)/ml), enterococci (10(5)) and Bacterioides fragilis (10(6)) were always present. Blood cultures obtained during this phase were uniformly positive, with E. coli being the principal isolate. Animals that survived this early acute peritonitis stage developed indolent intra-abdominal abscesses. The major isolates in abscess contents were B. fragilis (10(8.7)) and Fusobacterium (10(8.6)); E. coli (10(7.8)) and enterococci (10(5.7)) were also present but in lesser concentrations. Rank order analysis of these four species in peritoneal exudates and abscess pus showed that the two aerobes outranked the two anaerobes during the early stage of the disease, whereas the reverse was true in abscesses. These experiments also illustrated that a major simplification of the original fecal inoculum occurred, even though the subsequent infection remained bacteriologically complex.