Since the wound is the most common focus of infection in the surgical patient, adequate levels of antibiotic within the wound ar essential. This study examines the concentrations of antibiotic achieved in human wounds. Fluid was collected at timed intervals on the first postoperative day from the wounds of 56 patients receiving antibiotics after regional lymph node dissection. Antibiotic concentration was determined by bioassay. Six antibiotics were studied: cephalothin, cefazolin, cephapirin, oxacillin, ampicillin and clindamycin. The cephalosporins and penicillins showed similar patterns of appearance in the wound fluid. The peak level occurred early (1--1 1/2 hours) with subsequent slow decrease. Clindamycin produced nearly constant levels in wound fluid. The concentration of each antibiotic in wound fluid surpassed the serum levels after 2.5 hours. At the dosages studied each antibiotic produced wound fluid concentrations greater than the MIC for most susceptible organisms. Higher doses provided higher wound fluid levels. The rate of appearance and the levels achieved should be considered in the choice of antibiotics in the surgical subject.