The technetium(V) complex [(TcO)-Tc-99g(MAG(3))](-) [MAG(3) = mercaptoacetylglycylglycylglycine) can be prepared by substitution of the free ligand onto [(TcOCl4)-Tc-99g](-). The product is characterized by UV-visible spectroscopy, FAB mass spectrometry, H-1 and C-13 NMR spectroscopy, and X-ray crystallography. (Ph(4)As)[(TcO)-Tc-99g(MAG(3))], chemical formula TcAsSO6N3C32H29, crystallizes in the monoclinic space group P2(1)/n with Z = 4 and lattice parameters a = 12.252(6) Angstrom, b = 10.169(6) Angstrom, c = 24.579(7) Angstrom, beta = 92.04(3)degrees, and V = 3060(2) Angstrom(3). The final R value is 0.042. The technetium coordination geometry is square pyramidal with Tc=O = 1.647(3) Angstrom, Tc-S = 2.279(2) Angstrom, and Tc-N = 1.987(15) Angstrom on average. The MAG(3) ligand is usually prepared in a form wherein the thiol functionality is protected by a benzoyl group. Preparation of the [(TcO)-Tc-99m(MAG(3))](-) renal function imaging radiopharmaceutical by the Sn(II) reduction of [(TcO4)-Tc-99m](-) in the presence of benzoyl-MAG(3) is also described. HPLC comparisons verify that the two products [(TcO)-Tc-99g(MAG(3))](-) and [(TcO)-Tc-99m(MAG(3))](-) are chemically identical. The methyl ester derivative of MAG(3) forms a stable complex with the TcO3+ core and [(TcO)-Tc-99g(MAG(3)OMe)](-) is shown by HPLC to be a side product generated during the formation of [(TcO)-Tc-99g(MAG(3))](-). [(TcO)-Tc-99g(MAG(3)OMe)](-) is characterized herein by its direct synthesis and structural characterization. Two different conformations of the ligand in (PPH4)[(TcO)-Tc-99g(MAG(3)OMe)]. 2H(2)O have been shown crystallographically : polymorph 1 crystallizes in the space group P (1) over bar with lattice constants a = 11.054(4) Angstrom, b = 12.583(5) Angstrom, c = 13.810(5) Angstrom, alpha = 68.79(3)degrees, beta = 76.15(3)degrees, gamma = 88.86(3)degrees, V = 1734(1) Angstrom(3) and Z = 2; polymorph 2 crystallizes in P (1) over bar with lattice constants a = 12.62(1) Angstrom, b = 12.87(2) Angstrom, c = 13.09(2) Angstrom, alpha = 117.5(1)degrees, beta = 111.19(9)degrees, gamma = 90.3(1)degrees, V = 1719(4) Angstrom(3), and Z = 2. Averaged bond lengths for [(TcO)-Tc-99g(MAG(3)OMe)](-) are Tc=O = 1.656(3) Angstrom, Tc-S = 2.289(3) Angstrom, and Tc-N = 2.00(3) Angstrom. The geometry about technetium is square pyramidal with insignificant chemical differences between the two polymorphs. The syntheses of the ligands MAG(3) and MAG(3)OMe are described.