Transient and distant infections alter later intraperitoneal abscess formation. Transient nosocomial infections, such as line sepsis and pneumonia, are common in today's critical care patient population. Although generally well treated, the effect of these transient antigen exposures on the immune system is unclear. We have previously shown that prior intraperitoneal inoculation with live bacteria leads to increased numbers of intraperitoneal abscesses. Data presented here demonstrate in a murine model that two immunizations with live Escherichia coli, Bacteroides fragilis, or both, administered systemically via intracardiac injection or at a focal distant site in subcutaneous tissue, significantly increased the number of mixed E coli/B fragilis intraperitoneal abscesses when induced 1 week later. Further, immunization with E coli, either alone or in combination with B fragilis, increased the total number of anaerobes recovered per mouse. Transient or focal sublethal infections can significantly alter an animal's immune response to later infectious insults, particularly the formation of intraperitoneal abscesses. 