Spiramycin is a new antibiotic produced by a strain of Streptomyces isolated from soil samples in France.1The product was isolated by the French firm, Rohne-Poulenc Company. Sharp & Dohme, Division of Merck & Co., Inc., is exploring its clinical utility in the United States and supplied the spiramycin used in this study. The product is primarily a mixture of three chemical compounds, which can be separated but have not yet been completely differentiated. It is supplied in 0.5 gm. tablets, is hygroscopic, and has a bitter taste. Decomposition occurs rapidly at a pH lower than 4 or higher than 8. This report concerns laboratory studies and a clinical trial with spiramycin in 29 adult patients with bacterial pneumonia. METHODS Sensitivity Tests. —Tests to determine the concentrations of spiramycin necessary to inhibit the growth of streptococci and pneumococci were carried out in tryptose phosphate broth containing 3% human blood.