The effect of streptomycin in retarding the growth rate of Mycobacterium tuberculosis was already demonstrated by Smith and Waksman¹⁾ and the fact is commonly known. However, the mcde of action of the drug on the growth rate of the organism appears to remain out of considerations of most investigators. The purpose of the present paper is, therefore, to informate the mode of action of streptomycin on the growth rate of M. tuberculosis.