Fosfomycin levels were determined in the serum and the lung tissue removed during 26 pulmonary operations following intramuscular or intravenous administration of 2 g fosfomycin. In the serum the level was about 32 micrograms/ml 1--2 h subsequent to the administration. At the same time, the removed lung tissue showed 12--16 micrograms/g concentration in the apparently intact parts, amounting to 32--52% of the serum level, whereas in the tumorous lung tissue roughly half of this concentration was present. The fosfomycin levels found in the lung tissue are effective only against a group of bacteria causing respiratory inflammation.