Twenty patients undergoing colorectal surgery were given an initial dose of 2 g latamoxef (moxalactam) parenterally at the induction of anaesthesia. Ten of the patients received two subsequent doses at 8 h intervals. A series of serum and faecal specimens were taken for analysis of latamoxef concentrations. Tissue samples from intestinal mucosa and peritoneal fat were also taken at surgery. The mean serum peak level was 119.8 +/- 10.7 mg/l. The mean half life was 2.8 h and the mean area under the serum concentration curve was 345.0 +/- 25.7 mg h/l. The concentrations in intestinal mucosa were high when the tissue was removed within 4 h after the administration of latamoxef (22.6 +/- 1.7 micrograms/g, mean +/- S.E.). The concentrations declined when the tissues were removed later (7.8 +/- 0.7 micrograms/g). The content of latamoxef in peritoneal fat varied, range 0.8-23.4, mean 8.5 +/- 1.4 micrograms/g. Measureable levels of latamoxef in faeces were found in all patients except one. The values varied between 0.2 to 23.0, mean 8.9 +/- 1.5 micrograms/g. 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, lactobacilli, bifidobacteria, clostridia, bacteroides and fusobacteria--decreased markedly during the same period. After two weeks the microflora was normalized in all patients. There were no differences between the patients receiving one dose of latamoxef and those receiving three doses. No postoperative infections occurred.