&NA;Concentrations of ampicillin after intraincisional instillation in laparotomy wounds were measured in ten patients undergoing appendectomy. Ampicillin, 1 gm, was instilled under the fascia and 1 gm in the subcutaneous space during wound closure. Wound secretion was collected every two hours during the first 24 postoperative hours by cannulation of a fine perforated drain placed in the subcutaneous space. Ampicillin was determined by a disk diffusion method. During the first eight hours the median concentration of ampicillin in wound secretion exceeded 1000 microgram/ml; 14 hours and 20 hours after wound closure the median concentrations were 73 and 14 microgram/ml, respectively. The effect of ampicillin in high concentrations on “resistant” strains ofBacteroides fragiliswas demonstrated in anin vitroexperiment. It is concluded that in colorectal surgery the effect on wound sepsis of intraincisional antibiotics as an addition to systemic antibiotic prophylaxis should be evaluated in a clinical trial.