The antibacterial cell-binding properties of lactoferricin B, a potent bactericidal peptide derived from bovine lactoferrin, were investigated for the first time. To facilitate measurements of binding the peptide was radiolabelled by reduction and treatment with iodo-[1-14C]acetamide. 14C-lactoferricin B bound rapidly to the surface of Escherichia coli and Bacillus subtilis VSports手机版. The rate of binding was consistent with the rapid rate of killing caused by this peptide. The extent of binding was reduced in the presence of Mg2+ or Ca2+ ions which act to reduce its antimicrobial effectiveness. The optimal pH for binding was strain-dependent and the killing effect was maximal near the optimal pH for cell binding with each strain tested. These observations indicate that direct interaction of lactoferricin B with the cell surface is necessary for its lethal effect. The number of peptide molecules bound (> 10(6) per cell) was more than would be expected for binding to specific protein receptors. Lactoferricin B inhibited bacterial uptake of 3H-proline with effectiveness similar to polymyxin B, a known membrane-disruptive agent. The cell-binding event appears to lead to a disruption of normal permeability functions of the cytoplasmic membrane. .