A prospective study was performed on 196 consecutive patients undergoing elective colonic surgery to evaluate the prophylactic effect of a single dose of doxycycline. The patients were randomized into four groups: group I 200 mg i.v. preoperatively, group III 600 mg i.v. preoperatively, group III 600 mg i.v. postoperatively, and group IV 200 mg i.v. preoperatively and 200 mg i.v. daily 3 days postoperatively. The rate of septic complications for the different groups were: I 13 per cent, II 7 per cent, III 20 per cent, and IV 19 per cent. There was no statistically significant difference in occurrence of septic complications between the groups. The degree of bacterial contamination during operation was estimated by culture from wound irrigation fluid and from cotton swabs. Bacteria were recovered from the irrigation fluid in 97 per cent, while culture from cotton swabs proved to be much less sensitive. A high number of bacteria in the irrigation fluid was significantly correlated to a high complication rate. It is suggested that direct plating from irrigation fluid can be used for defining a high risk group of patients in colonic surgery.