Serum concentrations of dibekacin (DKB), sisomicin (SISO) and gentamicin (GM) were measured in 3 rabbits after intratracheal administration through the transtracheal teflon tube. The peak serum levels (average) were 106 micrograms/ml (administrated 100 mg DKB for injection), 148 micrograms/ml (administrated 100 mg DKB solution), 166 micrograms/ml (administrated 100 mg SISO solution) and 80 micrograms/ml (administrated 80 mg GM solution). Serum concentrations and urine excretions of DKB were measured in 3 volunteers after aerosol administration using ultrasonic nebulizer. The peak serum levels (average) were 4.6 micrograms/ml (administrated 100 mg DKB for injection) and 3.1 micrograms/ml (administrated 100 mg DKB solution). The urine excretions (average) were 3.7 mg and 4.3 mg respectively during 6 hours. Before and after administration of DKB aerosol the spirogram and flow-volume curve were examined in the volunteers. But the examinations showed no changes. Sputum concentrations were measured in 1 patient with chronic bronchobronchiolitis after administration of DKB aerosols using the ultrasonic nebulizer. The highest sputum concentration was acquired immediately after nebulization and the sputum levels decreased gradually while time passed. Six patients with the lower respiratory tract infections were treated with DKB aerosol therapy and the utility rate was 80%.