The causative agents of localized peritonitis are certain microbial associations of 2-6 bacterial species including staphylococci, enterococci, Escherichia coli, Proteus, and obligate anaerobes Cl. perfringens. A dominant position is occupied by Escherichia coli and obligate anaerobes in critical doses (105 and more microbial bodies per 1 ml of exudate). In the course of the disease all five types of antibodies accumulated independently of one another, while the intensity of their rise depended on the term of treatment and the patient's age. A twofold and greater increase of the antibody titers may confirm the etiological role of the microbes in peritonitis, whereas the level of their increase may be an indirect criterion of localized peritonitis prognosis.