The fundamenal and clinical studies of Josamycin, a new antibiotics as a nember of macrolide family, in otolaryngological field were carried out and the following results were obtained. 1) The sensitivity of Staphylococcus aureus (60 strains), β-Streptococcus (3 strains), Enterococcus (9 strains) and Pseudomonas aeruginosa (9 strains), isolated from the clinical materials, to Josamycin were studied by means of agar plate dilution methoth. The most of tested strains of Staphylococcus aureus (56 strains, 93%) was inhibited to growth in MIC 1.6~3.2 mcg/ml. Some Strains of Erythromycin-resistance Staphylococcus were sensitive to Josamycin. All of 3 strains of β-Streptococcus were sensitive to Josamycin. Out of 9 strains of Enterococcus, only 4 strains were sensitive to Josamycin. All of 9 strains of Pseudomonas aeruginosa were resisted to Josamycin and Erythromycin. 2) Electron microscopid finding. In contrast to clear cell membrane of normal control group, the Josamycin treated Staphylococcus showed thickening of the cell membrance and increased Electron Transparent Area. Differentiation of intracellular structure was also difficult in Josamycin treated group. 3) The efficiency of Josamycin was evaluated in 59 patients with infection of ear, nose and throat and 20 patients who underwent E.N.T. surgical intervention. Salient therapeutic response was obtained in 77 patients (88%). Josamycin was particularly effective in therapy of acute infection of tonsils and larynx, and in prevention of post-operative infection with no single failure.