Septic complications of colonic perforation involve miltiple bacteria derived from the intestinal flora. This type of mixed intraabdominal infection was produced experimentally by insertion of a standardized inoculum of rat colonic contents into the peritoneal cavity of male Wistar rats. The respective roles of coliforms and anerobic bacteria were then studied by use of selective antimicrobial therapy (with gentamicin and clindamycin). Untreated rats had a two-stage disease. Initally, there was an acute peritonitis associated with a 37% mortality rate; all animals that survived developed indolent intraabdominal abscesses. Treatment with gentamicin reduced the acute mortality rate to 4%, but 98% of the survivors had abscesses. Clindamycin acute mortalherapy was associated with a 35% mortality rate, but the incidence of intraabdominal abscess was only 5%. A combination of gentamicin and clindamycin yielded the salutary effects of each agent--7% mortality and 6% incidence of abscesses. These studies, in concert with bacteriological findings, suggest that coliforms caused early mortality while anaerobes were primarily responsible for the late complication of intraabdominal abscess formation.