Liposomes are important carriers for controlled drug release, for gene or antisense therapy, and for immunization. Radiolabeled liposomes can be used to evaluate the in vivo behavior of different liposome formulations, as well as for diagnostic imaging and radionuclide therapy. A novel method for radiolabeling liposomes with 99m Tc-''SNS/ S'' complexes is introduced. This labeling method can be applied to liposome radiolabeling with not only 99m Tc but also two therapeutic radionuclides, 186 Re and 188 Re. Liposomes encapsulating glutathione (GSH) were studied for 99m Tc labeling. N,N-bis(2-mercaptoethyl)-N 0 ,N 0 -diethyl-ethylenediamine (BMEDA), N,N-bis(2-mercaptoethyl)-1-butylamine (BMBuA), and benzene thiol (BT) were investigated to make 99m Tc-''BMEDA'', 99m Tc-''BMEDA þ BT'', 99m Tc-''BMBuA'', and 99m Tc-''BMBuA þ BT'', for liposome labeling. The labeling efficiencies of 99m Tc-GSH liposomes were from 36.9 to 69.2%. After incubation in serum, 99m Tc-GSH liposomes labeled with 99m Tc-''BMEDA'' or 99m Tc-''BMEDA þ BT'' had the best labeling stability of the formulations tested. Distribution studies after intravenous injection of 99m Tc-liposomes composed of distearoyl phosphatidylcholine (DSPC) and cholesterol had a slow blood clearance and a high spleen accumulation demonstrating the in vivo labeling stability of the radiolabeled liposomes. The 99m Tc-liposomes have great potential as a radiopharmaceutical system for evaluating various kinds of liposomes with different lipid composition, for evaluating in advance a subsequent radionuclide therapy using 186 Re or 188 Re labeled liposomes and for diagnostic imaging.