Preoperatively, to prevent infection, seventy-one patients who were to have total hip arthroplasty were given one gram of cephalothin, cephradine, or cefazolin intravenously. Simultaneous samples of bone and serum were obtained after various time intervals and assayed for cephalosporin concentration to correlate the antibiotic concentrations in these sites with time. Of the cephalosporins tested, cefazolin achieved the highest total peak levels in bone (thirty micrograms per gram), followed in descending order by cephradine (twenty-three micrograms per gram) and cephalothin (2.8 micrograms per gram). These peak levels in bone, reached twenty-five to forty minutes after injection, were sixty, 6.7, and fifteen times higher than the usual mean minimum inhibitory concentrations of cefazolin, cephradine, and cephalothin, respectively, for penicillin-resistant staphylococci. The half-lives of the antibiotics in bone were forty-two, forty, and thirty minutes, respectively.