Cefbuperazone was given intravenously in a dose of 1 g at induction of anaesthesia, followed by a subsequent dose of 1 g 12 h after the first dose, to 10 patients undergoing colorectal surgery. A series of serum and faecal specimens were taken for analysis of cefbuperazone concentrations. Tissue samples from the gut wall were taken at surgery. The maximum serum concentrations (mean value 98.6 +/- 9.1 mg/l) during surgery were reached 30 min after cefbuperazone administration. The cefbuperazone concentration in the tissue samples varied between 7.3 and 40.5 mg/kg and the concentration in the faecal samples between 0 and 27.0 mg/kg. Faecal samples were also collected during the investigation period for cultivation of aerobic and anaerobic bacteria. The aerobic bacteria--streptococci, enterococci and enterobacteria--were suppressed significantly during the prophylaxis period. Among the anaerobic bacteria, cocci, bifidobacteria, eubacteria, lactobacilli, clostridia, fusobacteria and bacteroides decreased markedly during the same period. After four weeks the microflora was normalized in all patients. No postoperative infections occurred.