A controlled prospective trial to compare the efficacy of the antibiotics cephaloridine and flucloxacillin in preventing infection after total hip replacement was conducted at three hospitals. The antibiotic regimens began before surgery, cephaloridine being continued for 12 hours and flucloxacillin for 14 days afterwards. Over an 18-month period 297 patients undergoing a total of 310 hip replacements were entered into the trial and randomly allocated to one of the regimens. The follow-up period ranged from one to two and a half years. All operations were performed in conventional operating theatres; at two of the hospitals these were also used by various other surgical disciplines. Four patients developed deep infection, two having received the cephaloridine and two the flucloxacillin regimen. The overall rate of deep infection was therefore 1.3%. Thus three doses of cephaloridine proved to be as effective as a two-week regimen of flucloxacillin. Giving a prophylactic systemic antibiotic reduced the incidence of infection to a level comparable with that obtained in ultra-clean-air operating enclosures.