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A number of derivatives of mercaptoacetyltriglycine in which one or more of the three glycylmoieties are replaced by other amino acids such as D- or L-alanine, D-serine, D-2-aminobutyric acid, D-valine or D-phenylglycine have been synthesized and labelled with technetium-99m. Labelling with technetium yielded in each a mixture of mainly two radioactive species, which probably are diastereomeric oxotechnetium (V) complexes originating from the presence of a chiral carbon atom in the ligands. The diastereomers were separated by reversed phase HPLC. Except for the serine derivative, all radiolabelled derivatives have a longer retention time than the parent compound 99mTc-MAG3. The relative amount of the two diastereomers formed upon labelling is dependent on the nature of the ligand but can be influenced to a limited extent by the pH of the exchange labelling mixture.