It has been suggested that wound infection rates after colorectal operations are influenced more by the presence of adequate tissue levels of antimicrobials at the time of contamination than by the extent of bacterial colonization of the intestinal lumen. There are, however, theoretical grounds for the belief that both levels are important. The authors therefore conducted a random control trial in 119 consecutive patients undergoing elective colorectal operations, comparing the results in a group receiving purely parenteral antimicrobial prophylaxis with those in one having a combined oral and parenteral regimen. The results in 83 contemporary nonrandomized patients, all of whom had the combined prophylactic regimen, are also reported. Oral bowel preparation resulted in a significantly smaller number of operation cultures showing growth of fecal gram-negative aerobes and anaerobes than did the purely parenteral regimen. There were more isolations of enterococci in the combined group but the excess did not achieve statistical significance. The rates of infective complications were higher in the parenteral than in the combined group, the difference in wound infection rates being statistically significant; the figures were 27.6 percent and 13.9 percent, respectively (P = .04). It is concluded that, in colorectal operations, it is advisable not only to ensure adequate tissue levels of antimicrobials but also to reduce the risk of endogenous bacterial infection by partially decontaminating the bowel.