The disulphide bridges of the Fc fragment (C-terminal half of the heavy chain) have been studied in several human immunoglobulins, containing heavy chains of different antigenic types (gamma1, gamma2, gamma3 and gamma4), and in heavy-chain-disease proteins. Two intrachain disulphide bridges were found to be present. The sequences appear to be identical in the Fc fragments of two types of chain studied (gamma1 and gamma3), and very similar to corresponding sequences of the Fc fragment in rabbit. These results suggest that the C-terminal half of the heavy chains is covalently folded (in a similar fashion to the light chains) with a C-terminal loop and an N-terminal loop. The similarity is emphasized by comparison of the sequence and location of the disulphide-bridged peptides of the C-terminal loop of heavy and light chains. The N-terminal loop, on the other hand, appears to be very different in Fc fragments and light chains. The C-terminal loop is the only one present in the F'c fragment.