S-Benzyl-, S-benzamidomethyl- and S-benzoylmercaptoacetyltriglycine were synthesized and compared in exchange labelling experiments for the preparation of 99mTc-MAG3. The rate of exchange from 99mTc-tartrate to 99mTc-MAG3 starting from the respective precursors was determined in different conditions. Labelling proceeded most rapidly starting from the S-benzoyl protected precursor but efficient labelling was also accomplished using the more stable S-benzamidomethyl- and S-benzylmercaptoacetyltriglycine. 99mTc-MAG3 was also prepared by direct labelling of unprotected mercaptoacetyltriglycine at alkaline pH. Radiochemical purity in these conditions is mainly dependent on the pH during labelling.