In an open, prospective, non-randomised study involving 112 patients undergoing elective colorectal surgery, the effect of perioperative prophylaxis with cefotetan was investigated. Cefotetan (2g) was administered, pre- and intra-operatively only. Preoperative bowel preparation was done by the standardised "Würzburg Method" i.e. oral metronidazole pre and post orthograde lavage. Mucosal biopsies were obtained from the resected colon and simultaneously serum samples were taken to determine tissue and serum levels respectively. Antibiotic serum and gut mucosal levels were well in excess of the minimum inhibitory concentration (MIC90) levels of the isolated bacteria. Wound infections occurred in only 2 patients. Cefotetan was well tolerated and no adverse events were noted. In prolonged colorectal surgery, an antibiotic such as cefotetan with a long half-life is to be recommended.