Site‐directed mutagenesis has been used to change amino acid residues of a recombinant Fc‐hinge fragment derived from the murine immunoglobulin (Ig)G1 molecule, and the effects of these mutations on the pharmacokinetics of the Fc‐hinge fragment have been determined. Specifically, Ile‐253, His‐310 and Gln‐311 of the CH2 domain and His‐433 and Asn‐434 of the CH3 domain have been changed. In the three dimensional structure of an antibody, these amino acids are in close proximity to each other at the CH2‐CH3 domain interface. The mutated Fc‐hinge fragments have been purified from recombinant Escherichia coli cells and their pharmacokinetic parameters determined in mice and compared with those of the wild‐type Fc‐hinge fragment. The results show that the site of the IgG1 molecule that controls the catabolic rate (the ‘catabolic site’) is located at the CH2‐CH3 domain interface and overlaps with the Staphylococcal protein A binding site.