Specific receptors for lactoferrin (Lf) have been identified in the small intestine brush-border membrane and on lymphocytes. Little is known about the mechanism of Lf interaction with its receptor protein. Because of the pH dependence of this interaction, the possible involvement of surface exposed histidine residues was explored. His residues on purified human milk Lf were progressively modified by carboxyethylation with diethyl pyrocarbonate; the reaction was monitored by UV absorbance and by affinity chromatography on immobilized Cu(II) ions. Human infant brush-border membrane vesicles (BBMVs) were prepared by differential centrifugation and precipitation methods. Human peripheral lymphocytes were prepared by Ficoll centrifugation and stimulated by mitogen (PHA) exposure in culture. A competitive receptor binding assay was performed with 59Fe- and/or 125I-labelled native Lf and His-modified Lf. Binding assays with labelled native Lf were conducted in the presence of a 10-200 fold molar excess of DEPC-Lf, the DEPC-Lf was not competitive. A 100-fold molar excess of native Lf completely blocked the interaction of labelled Lf with the receptor. Similar results were obtained for receptor proteins on BBMVs and lymphocytes. Thus, Lf receptors on these two different cell types appear to require His residues for Lf binding.