The binding of recombinant wild type and mutant Fc-hinge fragments to soluble, FcRn expressed in insect cells has been analysed. The mutant Fc-hinge fragments are derived from murine IgG1 with mutation of residues located at the CH2CH3 domain interface (Ile253, His310, Gln311, His433 and Asn434; EU numbering). These mutant Fc-hinge fragments have previously been shown to be deficient in neonatal transcytosis in suckling mice and also have abnormally short serum half lives. The mutated residues are highly conserved in human and rodent gammaglobulins (IgGs) and are also involved in binding to staphylococcal protein A. This study demonstrates that the Fc mutants have lower binding affinities for recombinant FcRn and mutations in the CH2 domain have a greater effect than those in the CH3 domain. There is an excellent correlation between affinity and transcytosis or the control of catabolism, and this provides further evidence in support of the close overlap of the sites of IgG Fc involved in these processes. The stoichiometry of the FcRn:Fc interaction has also been investigated and has been found to be 1:1, indicating that binding of FcRn to one CH2CH3 domain interface site precludes an FcRn:Fc interaction at the second site.