Liver abscess was induced in rats by injecting a culture of B. fragilis ss. fragilis (approx. 5X10(6) cells) into devascularized liver tissue. Mixed infection with B. fragilis and aerobes occurred in all animals subjected to repeated laparotomies for bacteriological sampling (4, 7, 14 and 28 days after inoculation). In animals sampled only at 8 or at 42 days after injection, B. fragilis was isolated in pure culture from the majority of animals. The size of abscesses containing B. fragilis in pure culture decreased markedly between 8 and 42 days after inoculation. In inoculated animals and in sham-operated animals with secondary infection histological examination revealed an abscess containing necrotic liver tissue and numerous granulocytes. In sham-operated animals with no bacterial growth the histological examination microscopy revealed a liver infarct with incipient organization and no accumulation of granulocytes.