In a prospective randomized double-blind trial the effect of a short prophylactic course of a systemically administered antibiotic combination was studied in a series of 53 consecutive patients undergoing elective colorectal surgery. Bowel preparation was performed conventionally with cathartics and saline enemas or with an elementary diet. Patients of both preparation groups were randomly divided into a treatment group and a control group. Treated patients received 3 doses of kanamycin 500 mg plus clindamycin 600 mg intramuscularly. Four patients were withdrawn from the study for various reasons, leaving 49 patients for analysis. Wound infections were regarded as slight (grade I) or clinically important (grade II). The frequency of all wound infections (grade I + grade II) was 8% in the treatment group (25 patients) versus 66.7% in the control group (24 patients). The frequency of the grade II wound infections alone was 4.0% versus 54.7%. The difference was significant in both cases (p less than 0.0005).