A model of intraabdominal sepsis in the tumour-bearing host was established in order to study the interactions between host, tumour and infecting organisms. BALB/c mice bearing a transplanted tumour were given an intraperitoneal inoculum containing Bacteroides fragilis, Escherichia coli and bran as an abscess-potentiating agent. Tumour-bearing mice formed abscesses which were not significantly smaller than in controls except late in tumour growth. The bacterial contents of the abscesses were not significantly different to controls. In contrast, mice given an abscess-inducing mixture at or near the time of tumour cell inoculation had tumours which were significantly larger than in controls. The mechanism of tumour enhancement is not known.