A study on 42 surgical patients was carried out to find out whether cefuroxime may be substituted for gentamicin in combination with metronidazole in the treatment of peritonitis secondary to perforation of appendix. All patients recovered and there were four wound infections in each group. Both aerobic and anaerobic bacteria were cultured in the peritoneal fluid in 69% of cases and anaerobes only in 19% of the patients. Postoperatively the patients were followed up for one month. The mean length of the hospital stay and convalescence did not differ significantly. There was also no difference in the time to the removal of gastric suction, laboratory measurements and the incidence of pyrexia between the study groups. Although the number of patients was limited the present study indicates that gentamicin may be replaced by less toxic cefuroxime. This is especially the case if there are several risk factors in the use of gentamicin and if there is not a possibility for monitoring the serum levels of gentamicin.