A double-blind prospective study involving 1,591 clean orthopaedic surgical procedures was performed to test the effectiveness of preoperative and intraoperative antibiotics in reducing the postoperative infection rate. The antibiotic and placebo groups were analyzed for factors known to predispose to infection. A decrease in the over-all postoperative infection rate from 5 per cent in the placebo group to 2.8 per cent in the antibiotic group was found.