Patients with bronchopulmonary infections were treated with cefoperazone (CPZ), and the serum and pleural effusion concentrations were determined after 2g CPZ drip infusion. The following results were obtained: Eight of 10 patients treated with CPZ responded with a significant clinical improvement. Side effects were found in 4 cases; eruption in 1 case, fever and granulocytopenia in 1 case, elevation of GPT in 1 case, and thrombocytopenia in 1 case. But these side effects disappeared immediately after cessation of CPZ treatment. Intravenous drip infusion of 2 g CPZ yielded a peak serum concentration of 112.0 --210.0 micrograms/ml immediately after the end of drip infusion, and a peak pleural effusion concentration of 8.8--43.0 micrograms/ml at 2--6 hours after the end of drip infusion. The ratio of peak pleural effusion concentration to peak serum concentration was 4.4--26.9%.