Antibodies are labelled routinely with direct labelling techniques in which some of the native disulphide groups are reduced to sulphydryls. Using polyacrylamide gel electrophoresis (PAGE) as an analytical tool and human polyclonal IgG (HIgG) labelled with 99Tcm by the ascorbic acid (AA), stannous chloride (SnCl2) and 2-mercaptoethanol (2-ME) techniques, we have quantitatively determined any fragmentation of the protein and the radioactivity associated with fragments. HIgG labelled with 99Tcm by two bifunctional chelating agents provided the vital comparison. The results indicated that in the bifunctional chelating agent (BFCA) methods 65 and 61% of the activity was associated with protein with an apparent molecular weight of 150 000 D. The corresponding numbers for the AA, SnCl2 and 2-ME methods were 46.4, 21 and 3.9%. With 2-ME, 96% of the activity was associated with low molecular weight fragments. Although monoclonal antibody fragmentation may not affect immunoreactivity of the protein, it may influence biodistribution.