We have studied the concentrations of ciprofloxacin in serum, bronchial mucosa, lung parenchyma, and pleural tissue after a single intravenous dose of 200 mg in 20 patients subjected to lung surgery. The concentrations of ciprofloxacin in the tissues exceeded that in the serum by 3-fold to 7-fold: serum 0.6 micrograms.ml-1, bronchial mucosa 1.9 micrograms.g-1, lung parenchyma 3.4 micrograms.g-, and pleural tissue 1.7 micrograms.g-1. The achievable concentrations of ciprofloxacin in the tissues of the lower respiratory tract are above the MICs for most lung pathogens.