Subbacteriostatic, growth-inhibitory concentrations of potassium sorbate (125, 250, and 500 [mu]g/ml) at pH 5.70 increased the rate of proton translocation, and strongly inhibited the transport of arginine, glutamic acid, phosphate, and sulfate by whole cells of E. coli K12. Potassium sorbate (125 [mu]g/ml) had almost no effect on the uptake of glucose, thymine, or uracil by E. coli 15TAU or E. coli K12 but effectively inhibited the synthesis of DNA, RNA, and protein in both organisms. All three concentrations of sorbate inhibited respiration (oxygen consumption) in E. coli K12 but failed to induce leakage of [superscript]14C-labeled intracellular materials from radiolabeled cells of that same organism. There was a relatively small difference (8%) in the inhibition of glutamate and arginine uptake in whole cells of E. coli K12 by potassium sorbate (125 [mu]g/ml);The results of the present study suggest that potassium sorbate increases the permeability of E. coli cells to protons and probably other small ions thus decreasing the proton gradient as well as the charge difference across the cytoplasmic membrane. This action can diminish the capacity of the cytoplasmic membrane to function in the active transport of nutrients such as amino acids, and inorganic ions essential to the growth of the bacterial cell.