Colicin Ia has been iodinated with [125I]iodine. Iodinated colicin retains biological activity and has a specific activity of approximately 106 to 107 cpm per nmole. At saturation, approximately 5000 Ia molecules are bound to an Escherichia coli cell. Scatchard plots revealed a heterogeneity in the Kassoc between colicin and receptors. The binding of 125I-Ia to biologically active receptors has an average Kassoc of approximately 1 x 1010 m-1 at 37°. Strains selected for resistance to colicin Ia were defective in their ability to bind colicin Ia or Ib. Furthermore, both colicins competed for the binding of 125I-Ia to bacterial receptors. These data confirm previous results suggesting that colicins Ia and Ib adsorb to common receptors. Colicinogenic cells were found to be indistinguishable from noncolicinogenic cells in both the amount of colicin adsorbed per cell and Kassoc. This suggests that mechanism of colicin immunity does not involve an alteration in the binding reaction.