We report the interim results of two trials to evaluate the place of mezlocillin in elective intestinal operations. Sixty-four operations for gastro-oesophageal disease have been performed where patients were randomly allocated to mezlocillin or cefuroxime. Wound sepsis occurred in 19% of the mezlocillin patients compared with 3% of those receiving cefuroxime. Seventy-three operations have been performed for colorectal cancer in whom three doses of antibiotic were used for prophylaxis. Patients were randomly allocated to mezlocillin and metronidazole or cefuroxime and metronidazole. The rates of abdominal wound sepsis in the groups were 15% and 12% respectively. Post-operative Clostridium difficile colitis occurred in four of the cefuroxime patients, compared with none of those receiving mezlocillin. Mezlocillin appears to be a safe and effective antibiotic when used in combination with metronidazole in colorectal resections for cancer.