For successful treatment of bacterial lung infections the administered antibiotic must reach sufficiently high concentrations in lung tissue. Therefore, the concentrations of enoxacin in this tissue were measured in ten patients requiring pulmonary surgery. In order to prevent postoperative infection, the patients received 400 mg enoxacin b.i.d. for three days. Eight h after the final dose samples of venous blood were drawn and a sample of lung tissue was removed. Using a microbiological assay, we found the following concentrations (mean +/- S.D.):serum 2.36 (+/- 0.65) mg/l, lung 6.48 (+/- 1.54) mg/kg. With the HPLC-technique the corresponding values method were 2.37 (+/- 0.80) mg/l and 7.41 (+/- 3.01) mg/kg. Thus concentrations of enoxacin in lung tissue are about three times higher than the corresponding serum concentrations.