Envelope fractions isolated from Escherichia coli K-12 C600 and from colicin-resistant and colicin-tolerant (Tol II) mutants derived from this strain were separated on sucrose gradients into cell wall-enriched and cytoplasmic membrane-enriched fractions. These fractions were tested for their ability to neutralize colicins of the E and K groups. Neutralization activity was found in the cell wall-enriched fraction from the parent and the Tol II mutant but was absent from all fractions from the resistant mutant. This was also tested with several other E. coli strains. In all cases, sensitive strains contained the neutralization activity, whereas resistant strains did not. The neutralization activity was solubilized from cell walls or cell envelopes of sensitive or Tol II strains by extraction at room temperature with Triton X-100 plus ethylenediaminetetraacetic acid. The solubilized activity was precipitated by 20% ammonium sulfate, 70% ethanol, or 10% trichloroacetic acid. The activity was destroyed by treatment of the solubilized preparation with trypsin or periodate. These results suggest that this colicin-neutralization activity is due to the presence of specific receptors localized in the cell wall and that intact protein and a carbohydrate are required for this receptor to bind colicin.