Doxycycline was used to prevent infection in 40 patients undergoing oesophageal surgery (for carcinoma in 35 and for benign lesions in 5). Treatment began with induction of anaesthesia and continued in doses of at least 200 mg/day for a mean period of 10 days. Concentrations of the antibiotic were measured in the serum and colonic tissue of 20 patients, 2 hours on average after the first injection. Mean serum and tissue levels were 4.1 +/- 0.8 mcg.ml-1 and 2.9 +/- mcg.ml-1 respectively, with a tissue to serum ratio of 0.7. This ratio remained practically unchanged between the first and fourth hours post-injection. Doxycycline was successful in preventing post-operative infection in 75% of the cases. No infection due to anaerobic organisms was observed.