Since a published report,1together with unpublished work by ourselves and our associates, demonstrated that streptomycin exerted both bacteriostatic and bactericidal effects on Bacterium tularense in low concentrations in vitro, as well as definite curative properties for the experimental disease in rodents, this study was undertaken to determine the effectiveness of streptomycin therapy in human tularemia. Seven patients were treated. Each had been infected by handling wild rabbits. All presented symptoms typical clinically of tularemia, and each developed specific serum agglutinins to 1:320 or higher. The causative agent was cultivated after mouse inoculation of primary lesion exudate from the first, third, fourth and fifth patients. The days of disease on which streptomycin therapy was instituted were the eighth, twenty-third, third, seventeenth, twentieth, fiftieth and one hundred and third respectively. EVIDENCES OF MODIFICATION OF THE NATURAL COURSE OF DISEASE The best