The results of two prospective, randomized trials, comparing single dose piperacillin with multidose cefoxitin prophylaxis in elective surgical procedures of the gastrointestinal tract were combined to examine some basic aspects of clinically, applied antibiotic prophylaxis. As there was no difference in the efficacy of prophylaxis between the groups, data from both centers and both drug groups were combined for analysis. Patients whose ages were greater than 40 years, the presence of cancer and the administration of an antibiotic for longer than 60 minutes preoperatively were associated with a higher rate of infectious complications. Duration of operation was also related to subsequent infection, ranging from 6 per cent in procedures lasting less than two hours to 27 per cent in operations lasting more than four hours. There was a greater failure of prophylaxis in patients undergoing rectal procedures, with a 29 per cent rate of infectious complications compared with 14 per cent in patients undergoing operations upon the colon. This difference was present even when duration of operation was taken into account.