Ribosomes recovered from cells of Escherichia coli treated with streptomycin are impaired in their response to the dissociation factor. This effect of Str evidently depends on a direct action on the ribosome and not on stabilization of a complex with normal ligands. Thus, such Str-ribosomes lack firmly bound transfer RNA; treatment with puromycin does not remove the resistance to dissociation; and similar resistance is produced when free ribosomes (in the absence of normal ligands) are exposed to Str in buffer and then washed. The impairment of dissociation of Str-ribosomes in cells is evidently incomplete, for these ribosomes maintain a reduced polysome level by engaging in cyclic abortive reinitiation (see preceding paper, Wallace & Davis, 1973) and this process requires formylation (and hence presumably dissociation). Str-inhibited dissociation may be the limiting step in this reinitiation, for the polysome level is much lower in Str-treated cells of strain W than in those of K12, and Str impairs dissociation much more with ribosomes of the former strain.