Prophylactic antibiotics should be given as close to the time of incision as possible to ensure that tissue antimicrobial levels are adequate and maintained for the duration of the procedure. The choice of antibiotic should be based on the organisms most likely to be encountered--usually staphylococcal skin florae. The choice of vancomycin over a cephalosporin may be justified in patients who are known carriers of MRSA. A full therapeutic dose of antibiotic should be used for prophylaxis. Morbidly obese patients should be given twice the standard dose. Redosing during an operation is recommended if the duration of the procedure exceeds two half-lives of the antibiotic administered. Prophylactic antibiotics should not continue to be administered more than 48 hours postoperatively.