Perioperative antibiotic administration to produce systemic effects reduces the frequency of operative wound infection in selected procedures. Controlled clinical trials now indicate that cephalothin sodium may not be effective in this role, whereas cephaloridine and cefazolin sodium have been beneficial. Review of estimates of incisional antibiotic concentrations taken during prospective patient studies display patterns compatible with these data. EAch drug reaches acceptable incisional concentrations, which are relatively well maintained for cephaloridine and cefazolin; cephalothin disappears from human incisions so rapidly as to provide little wound protection in operations lasting more than one hour.