Preexposure of the peritoneum to live bacteria increases later mixed intraabdominal abscess formation and delays mortality. Intraabdominal infections are a major source of morbidity and mortality for the trauma and postoperative patient. Transient peritoneal contamination with bacteria after either intentional or unintentional violation of the gut are common. The effect of this intermittent antigen exposure upon later formation of intraabdominal abscesses is unclear. Previous experiments by others have demonstrated that repeated exposure to Bacteroides fragilis capsular polysaccharide can induce a T lymphocyte-mediated immunity to subsequent induction of pure B. fragilis abscess formation. In a murine mixed intraabdominal abscess model, preexposure to live Escherichia coli, B. fragilis, or both increased the number of later abscesses and in some cases their bacterial composition. Further, immunization with E. coli alone increased late mortality without altering overall mortality. These data suggest that the alterations of immune function produced by live, transient bacteria upon subsequent mixed intraabdominal abscess induction result in fundamentally different consequences from those observed after specific polysaccharide antigen exposure and subsequent monomicrobial abscess induction. 