An IgG1K monoclonal component with abnormal covalent H and L chains structure (LIA protein) was identified during a systematic screening of myeloma proteins by means of non-reducing/reducing SDS-polyacrylamide gel electrophoresis. Using immunochemical and immunogenetic analysis the mutation was characterized as a hinge region deletion, with loss of L-H and H-H disulphide bridges and direct L-L bonds. Moreover, non-expression of the G1m(z) allotype suggested that the deletion might start at residue 216, a preferential site previously observed in other HCD proteins. This feature is in agreement with the discontinuous structure of immunoglobulin CH genes and suggests that an abnormal switch mechanism is responsible for the deletion.