As an adaptation to the iron-limited environment of the host, Haemophilus influenzae has a transferrin receptor-mediated mechanism of iron acquisition such that it can acquire iron directly from human transferrin. The absence of detectable siderophore production and the presence of transferrin binding in a collection of type b and nontypeable H. influenzae strains indicate that the mechanism is widespread in this species. Growth and binding studies have consistently shown that the receptor is specific for human transferrin, which correlates with the host range of this pathogen. Inhibitor experiments indicate that iron regulation of receptor activity is mediated at the gene level. Affinity isolation experiments indicate that, as observed with other bacterial pathogens, the receptor is composed of two iron-repressible outer membrane proteins, transferrin binding proteins 1 and 2.