Mutants of Escherichia coli, harbouring the uncA401 or uncB402 alleles, were found to take up streptomycin more rapidly than the coupled parent strains. The increased rate of uptake results in greater sensitivity of the uncoupled strains, compared to the parent strains, to low concentrations of streptomycin. Studies with unc+ revertants showed that hypersensitivity to streptomycin is attributable to the mutation causing uncoupling. The uptake of streptomycin in an unc- strain is abolished by addition of the chemical uncoupler carbonylcyanide m-chlorophenylhydrazone. The phenotype of hypersensitivity to streptomycin can be used as a selection procedure for the isolation of uncoupled strains. In an experiment reported here, nine out of 12 strains isolated as being sensitive to streptomycin (at 2.5 micrograms/ml), were found to be unable to grow on succinate as a sole source of carbon. Five of the nine Suc- strains were found to be uncoupled in oxidative phosphorylation, and two of the five uncoupled strains lacked Mg2+-ATPase activity. The mutations causing uncoupling were cotransducible with the ilv genes.