Effective concentrations of antibiotic in the fluid bathing implanted hip prostheses are essential to prevent infection by micro-organisms. Twenty patients undergoing total hip replacement were given one gram of Cephradine intramuscularly one hour before operation and one other received a single bolus of Cephradine intravenously before operation and one other received a single bolus of Cephradine intravenously before operation. The concentrations of antibiotic were greater and persisted longer in the tissue fluid than in the blood. The antibiotic was sufficient to inhibit most micro-organisms causing contamination. We recommend that Cephradine is given intramuscularly one hour before operation and at six-hourly intervals after operation until the drainage tubes and intravenous lines have been removed.