In a prospective study of cephalothin prophylaxis for patients who underwent aortocoronary bypass an unacceptably high rate (44%) of Staphylococcus aureus sternotomy infections occurred in a placebo-treated group. In two other groups of patients, one group given cephalothin intraoperatively and the other given the antibiotic both intra- and postoperatively, such infections occurred with similar frequency (2.6% and 2.1% respectively). This study demonstrates the need for antistaphylococcal agents during aortocoronary bypass operation. No advantage is derived by extending this therapy beyond the operative period.