In a sensitive strain of Escherichia coli, lethal concentrations of streptomycin lead to the accumulation of 70 s ribosomes (str-monosomes) that are incapable of protein synthesis either in vivo or in vitro. The accumulation of the str-monosomes takes place at the expense of polyribosomes as well as 50 s and 30 s ribosomes, and its rate is dose-dependent. At a streptomycin concentration of 75 μg/ml., the rate is 1.3% of all ribosomes per minute. Newly synthesized and pre-existing mRNA (measured by DNA-RNA hybridization) binds irreversibly to the accumulating str-monosomes. The amount of mRNA that is bound to the str-monosomes increases with time in proportion to the rate of monosome accumulation. The mRNA extracted from the monosomes is comparable in size to mRNA extracted from untreated cells, suggesting that it is stabilized from degradation on the str-monosomes. Monosomes can be formed in vitro from 30 s and 50 s ribosomal particles in the presence of streptomycin; and, as in the absence of the drug, their formation is dependent on K +, Mg 2+, mRNA and tRNA. Cells starved for their carbon source are neither killed by streptomycin, nor do they accumulate monosomes, showing that continued metabolism is required for monosome formation. We suggest that the binding of streptomycin in the aberrant monosomes is irreversible under physiological conditions, and that this binding accounts for the bactericidal action of the drug.