The concentration of CZON was determined by HPLC in surgical patients with chronic otitis media, sinusitis, and tonsillitis. One gram of CZON was injected intravenously prior to surgery. The time course of the mean tissue CZON level was as follows: In the middle ear mucosa, 3.7 micrograms/g at 15 min, 7.2 micrograms/g at 30 min, and 2.9 micrograms/g at 1 hr (the half life: 21.3 min). In the maxillary sinus mucosa, 10.5 micrograms/g at 15 min, 11.8 micrograms/g at 30 min, and 2.8 micrograms/g at 1 hr (the half life: 17.5 min). In the tonsils, 14.9 micrograms/g at 15 min, 9.3 micrograms/g at 30 min, and 2.0 micrograms/g at 1 hr (the half life: 13.2 min). The concentration was high in the maxillary sinus mucosa and the tonsils, but was low in the middle ear mucosa. In the formers the transfer ratio reached its peak 15 to 30 min after administration, but in the latter the peak was reached 30 to 60 min after administration. The order of the transfer ratio at each region was above 25%. The tissue concentration exceeded the MIC80s of frequent isolates from these infections. CZON is considered to be a highly useful drug in the treatment of these infections.