Expression of the meningococcal transferrin receptor, detected by assay with human transferrin conjugated to peroxidase, was regulated by the level of iron in the medium. The transferrin receptor was identified by SDS-PAGE and Western blot analysis, as a 71,000 molecular weight iron-regulated outer membrane protein in Neisseria meningitidis B16B6. Growth studies with iron-deficient cells and competition binding experiments demonstrated that the meningococcal receptor was species-specific for human transferrin. Reciprocal competitive binding experiments and limited proteolysis of intact cells indicated that the transferrin and lactoferrin receptors are distinct.