From Oct. 1989 to Apr. 1990, 16 patients with colorectal carcinoma undergoing elective radical resection were randomly divided into two groups to receive oral and combined (oral + i.v.) antimicrobials respectively. Patients in the oral group received preoperative oral gentamicin and metronidazole for two days, in the combined group oral medication was followed by the same antimicrobials intravenously during perioperative period. Quantitative bacterial cultures were performed before and after the regimen. Results showed that the preoperative prophylaxis with oral antimicrobial resulted in a significant reduction in the bacterial counts of the rectum contents in all the patients (P < 0.001). The NICs of gentamicin and metronidazole for E. coli and B. fragilis were found to be 2.31 micrograms/ml and 0.66 micrograms/ml respectively. The perioperative blood samples and the intraoperative tissue specimens were taken for gentamicin and metronidazole determination. In the oral group, effective concentration of metronidazole were found in serum and tissues but gentamicin was undetected. In the combined group, effective concentrations of both gentamicin and metronidazole were detected. According to our results, the short-term preoperative oral medications combined with perioperative intravenous gentamicin and metronidazole prophylaxis appears to rational.