Cefoxitin was administered intravenously to 11 patients undergoing colorectal surgery; the initial dose was 2 g given at the induction of anesthesia followed by two 2 g doses at intervals of six hours. Serum samples and faecal specimens were taken to determine the cefoxitin concentrations. Tissue samples from the gut wall were obtained at surgery. The serum concentrations 15 min after administration ranged from 106.6 to 664.0 mg/l [mean: 295.8 +/- 61.0 (SE) mg/l]. Cefoxitin concentrations in the faecal samples were between 0 and 57.7 mg/kg. Cefoxitin concentrations in the tissue samples varied from 9.0 to 68.3 mg/kg. Faecal samples were also obtained during the investigation period for the cultivation of aerobic and anaerobic bacteria. Enterobacteria decreased and enterococci increased during the first two days. Anaerobic cocci and gram-negative rods also decreased during the same period. The colonic microflora returned to normal after two weeks in all patients and no new colonizing cefoxitin-resistant bacteria were isolated. No postoperative infections occurred.