When used for immunoscintigraphic detection of malignant tumours, monoclonal antibodies are frequently labelled with indium-111. This study investigated whether antibody properties are affected by DTPA, the chelating agent with which the antibody has to be bound before labelling with indium-111. Chromatography, cell binding assays and animal studies indicated marked changes in the chemical, immunological and biological properties of the F(ab')2 antibody fragment 431/31 at DTPA:F(ab')2 ratios greater than 1. It is concluded that, in order to avoid lasting effects on the normal biokinetic behaviour of this antibody, the DTPA:F(ab')2 ratio for the antibody should not exceed 1.