The site of the murine IgG1 molecule that regulates catabolism has recently been shown to encompass amino acids that are located at the CH2-CH3 domain interface. The CH2 and CH3 domains are connected to each other by a relatively flexible "mini-hinge" region, and flexibility in this region could clearly affect the orientation of the domains with respect to each other. The internal movement of the CH2 domain depends on the absence/presence of the hinge disulphide. The increased mobility of the CH2 domain relative to the CH3 domain in a hinge less IgG or Fc fragment may result in a conformational change at the CH2-CH3 domain interface and alter the accessibility of the residues that are involved in catabolism control. To investigate this possibility, four Fc fragments which differ in the presence/absence of hinge disulphides and hinge sequences have been analysed in both in vivo pharmacokinetic studies and in vitro by limited proteolysis with pepsin. The data show that the presence of hinge disulphide(s) in the Fc fragment results in a longer intravascular half life but a higher susceptibility to pepsin attack. This, taken together with the knowledge that pepsin cleaves close to the CH2-CH3 domain interface, suggests that the longer half life of disulphide linked Fc fragments relative to unlinked fragments may be due to conformational differences in this region of the IgG molecule, and these conformational changes may affect the accessibility of the catabolic site for binding to putative protective Fc receptors.