A new approach to antibiotic prophylaxis was tested in a prospective study of postoperative septic complications in elective colo-rectal surgery. The degree of contamination in the operative field in a nontreated group was estimated by dipslide technique. When the slide yielded greater than or equal to 5 CFU of a coliform bacterium or S. aureus the patient was judged to be contaminated. This trait was significantly correlated to the development of postoperative infection. All contaminated cases in another group of patients were subjects to prophylaxis within 24 hours after peritoneal closure, using an antibiotic with a favourable MIC concerning the current bacterium (-iae). This resulted in a significant reduction of the rate of infection. These findings challenge the current concept of a very limited effective period of prophylactic systemic antibiotics.