Apo-transferrin (apo-hTf) and holo-transferrin (holo-hTf) were separately conjugated to 15-nm colloidal gold. Iron-restricted Neisseria meningitidis strain SD (B:15:P1.16) bound up to three-fold more holo-hTf than apo-hTf (p < 0.001). The ability of meningococcal mutants lacking either transferrin-binding protein A (TbpA) or TbpB to discriminate between apo-hTf and holo-hTf was also investigated. There was no significant difference between the amount of gold-labelled apo-transferrin bound by the isogenic TbpA mutant (expressing TbpB) and the parent strain, whereas an isogenic TbpB mutant (expressing TbpA) bound significantly less gold-labelled apo-hTf. The isogenic TbpA and TbpB mutants and the parent strain all bound significantly more holo-hTf than apo-hTf, whereas the double 'knock-out' mutant failed to bind hTf irrespective of the iron-loading. In the isogenic mutants, TbpB was more effective in binding either apo- or holo-hTf than TbpA. Monoclonal antibodies against TbpA and TbpB were used to co-localise the transferrin-binding proteins on strain SD. The ratio of TbpA:TbpB was approximately 1:1. TbpA and TbpB were occasionally observed in close proximity to each other, but the two proteins were generally quite separate, which may indicate that they do not usually form a complex to act as a transferrin receptor.