The influence of ceftriaxone on oral and intestinal flora was investigated in 10 patients undergoing colorectal surgery. Ceftriaxone was given intravenously in one 2g dose before anesthesia. Saliva and feces samples were collected and analyzed on day 0, 3, 5, 14 and 28 after drug administration. All specimens were cultured quantitatively for aerobic and anaerobic microorganisms; representative colonies of each morphologic type of organism cultured were identified. The oral aerobic flora was somewhat affected by the administration of ceftriaxone. In all patients the number of Streptococci, Staphylococci and Neisseria decreased during the 5 days after ceftriaxone administration. No significant changes in the number of anaerobic commensal occurred. The oral microflora of all patients after 14 days had returned to normal. The aerobic fecal flora was considerably affected: in all patients enterobacteria were eliminated or strongly suppressed. Only minor changes in the number of aerobic gram-positive bacteria were observed, and the anaerobic intestinal flora showed only minor alterations. On day 28 the intestinal flora were normalized in all respects. No new colonizing microorganisms were isolated during the investigation period and no colonization with ceftriaxone-resistant bacteria was observed. No postoperative infection occurred and no adverse effects were registered.