Experimental intra-abdominal abscesses were initiated by surgical implantation of a fibrin clot contaminated with either Bacteroides fragilis, Bacteroides thetaiotaomicron, or B fragilis-Escherichia coli. Seven days after surgery the numbers of bacteroides (per gram) in B fragilis and B thetaiotaomicron abscesses were typically log10 8.4 +/- 0.5 (n = 6) and log10 6.4 +/- 0.6 (n = 4), respectively; B fragilis-E coli abscesses typically contained log10 8.9 +/- 0.5 B fragilis and log10 7.6 +/- 0.6 E coli (n = 5). Of 38 B fragilis abscesses, 14 B fragilis-E coli abscesses, and nine B thetaiotaomicron abscesses, additional intestinal bacteria were recovered from 21 (55%), 13 (93%), and seven (89%) abscesses, respectively. The additional organisms, in decreasing order of frequency, were enterococci, E coli, staphylococci, alpha-streptococci, lactobacilli, and Proteus species in numbers ranging from 2.5 log10 to 7.9 log10 per gram of abscess. Histologic sections of contaminated abscesses adherent to the intestine, liver, or spleen revealed normal tissue histology and no breakdown of the abscess wall. Thus, intestinal bacteria translocated into intra-abdominal abscesses by a mechanism that did not appear to be surgical soilage.