Syntheses performed with unprotected mercaptoacetylglycylglycylglycine (MAG3) both at carrier level (10−4M) and with no-carrier-added99mTc indicated that various products of different biological properties exist in the Tc/MAG3 system. Besides the known Tc(V)oxo complex recently introduced into nuclear medicine for renal function diagnosis, mainly three species were characterized by HPLC, TLC, electrophoresis and uv-vis spectroscopy as well as biodistribution studies in minipigs. Alteration of preparation conditions such as concentrations, sequence of reactant addition, pH, and time enabled to elucidate reaction routes within the system, interdependence of the species, and vulnerability of the kidney imaging agent.