Surgery for perforated appendicitis was used to estimate the infective dose of aerobic and anaerobic bacteria in postoperative wound sepsis. The wound sepsis rates were 22.6% (7/31) after treatment with intravenous ampicillin sodium and metronidazole and 23.5% (8/34) after penicillin G sodium, streptomycin, and metronidazole, a nonsignificant difference. Intraoperative sampling by velvet pads demonstrated that the density of aerobes did not differ significantly from that of anaerobes, either on the surface of the appendix, in peritoneal exudate (aspirated), or in the wound before closure. The predominant pathogens were Escherichia coli and Bacteroides fragilis. In 15 patients who developed wound sepsis, the density of aerobes and anaerobes was significantly higher at all sampling sites than in 50 noninfected patients. The median infective dose of aerobes and anaerobes together was 4.6 X 10(5) colony forming units.cm-2 in the operative wound. There was a significantly high correlation between the densities of bacteria during operation and subsequent wound sepsis.