This study was undertaken t o characterize the kinetics of possible bacterial synergy using a mouse model of mixed intraabdominal infection with Bacteroides intermedius and Fusobacterium necrophorum. Female CD-1 mice were injected intraperitoneally with B. intermedius, P. necrophorum or mixtures of both organisms. Generalized septic peritonitis developed within 24 hr, with abscess formation occurring after one to two wk in survivors with the mixed infection. Involvement of the reticuloendothelial system was evidenced by dose-dependent hepatosplenomegaly, which appeared during the first wk postinfection and progressed throughout the course of the experiment. Indirect immunofluorescence confirmed the presence of both species of bacteria in frozen sections of liver tissue. The median lethal dose (LD,,) was 2.11 x 10s for the mixture, 3.03 x 109 for B . intermedius alone, and 1.07 x 109 for F . necrophorum alone. The median abscess-producing dose (AD,,), the dose required to produce abscesses in fifty percent of the surviving mice a t two wk, was approx. l / l O O of the LD,,dose. The AD,, for intrahepatic abscesses was 2.8 X lo8 for the mixture, whereas the AD,, for intraabdominal abscesses occurring in any site was 5.14 x 10'. Both Bacteroides and Fusobacterium persisted in tissue for a t least 22 wk following mixed infection. The persistence of the Bacteroides in tissue represents a synergistic result of mixed infection with FUSObacterium and contributed to the chronicity of intraabdominal abscess formation. Bacteroides, injected alone, did not produce abscesses at any of the doses tested. However, when passaged (isolated from mixed infection hepatic abscesses) B. intermedius was used, the bacteria did induce abscesses.