Cytological studies on the cellular components of streptomycin-sensitive strain B 3 of Bacillus suhtilis and its two resistant strains 2R 2 B 3 and 2R 3 B 3 reveal that development of streptomycin resistance is accompanied by an increase in both DNA and RNA content of cells. The resistant strains are able to concentrate in higher amounts 13 different amino acids in the free state in the internal environment. The levels of all the amino acids and amino sugars of cell walls are higher in the resistant strains. © 1968, Applied Microbiology, Molecular and Cellular Biosciences Research Foundation. All rights reserved.