A liposome-based preparation consisting of the beta- and gamma-emitting radionuclide rhenium Re 188 (Re 188) linked to the chelator N,N-bis (2-mercaptoethyl)-N',N'-diethylethylenediamine (BMEDA) and encapsulated in liposomes, with potential tumor imaging and antineoplastic activities. Upon intravenous infusion of rhenium Re 188 BMEDA-labeled liposomes, the liposomes selectively target tumor cells, facilitate the retention of the radioisotope by those cells, and cause localized antitumor radiocytotoxicity while sparing surrounding normal, healthy cells. In addition, Re 188 BMEDA-labeled liposomes can be used for imaging purposes. Re 188 has a short half-life and a short path length, which further contribute to limiting the radiotoxicity to the tumor cells.