Resistance to low (5 μg/ml) concentrations of streptomycin in agar media was not inherited by all of the surviving population. Outgrowth of cultures in liquid media supplemented with the antibiotic depended upon inoculum size. Antibiotic titers in the supplemented cultures decreased during incubation, and an inactive radioactive product was detected when [14C]streptomycin was used. This low-level resistance is, therefore, attributed to enzymic inactivation of the antibiotic. Growth in 10 μg/ml or higher concentrations of streptomycin on agar media was due to selection of resistant variants present in the parent strain. A range of such variants existed, decreasing in frequency as their degree of resistance increased. Examination of one that was resistant to moderate concentrations of streptomycin, (25 μg/ml) and a second that was resistant to high (100 μg/ml) concentrations of streptomycin suggested that both possessed ribosomes which had lower affinity for the antibiotic than those of the parent strain, and that tolerance to high levels of streptomycin was due to a resistant ribosomal system for protein biosynthesis.