The relative merits of different antibiotic regimens for prophylaxis in orthopaedic implant surgery are difficult to evaluate because of the low frequency of infection. Factors other than infection prevention may influence choice. We have compared 400 mg teicoplanin given intravenously on induction of anaesthesia with three perioperative injections of cefuroxime, in 146 patients undergoing total hip or total knee replacement. These interim results suggest that cefuroxime selects for increased extraintestinal carriage of faecal streptococci and teicoplanin for Proteus species. There were no significant differences between the regimen in the acquisition of coagulase-negative staphylococci or Clostridium difficile, post-operative diarrhoea, wound healing or wound infection. Both regimens were equally safe.