Radioactive therapeutic seeds include a substantially radiotransparent cylindrical capsule containing a radioactive isotope and, in particular embodiments, a radiopaque or MRI-visible marker. Each of the embodiments is designed to provide a substantially isotropic distribution of radiation. In one embodiment, the isotope is deposited on the outer surface of a hollow radiolucent tube and a collar is provided about a central portion of the tube. The capsule comprises two tubular halves positioned over the tube with the open ends of the halves forming an interference fit with the collar and welded thereabout. In another embodiment, the isotope bearing structure is a plurality of radiolucent spheres provided with a thin coating of silver to facilitate the adhesion of the isotope thereto. The spheres are provided in two capsule halves which are welded closed about a plug have an axial radiopaque marker therein. In yet another embodiment, the isotope bearing structure is a pair of silver tubes having surfaces on which the isotope is provided. One tube is positioned in each half of the capsule, and the halves are then welded about a plug provided with a marker. In each embodiment, the closed ends of the capsule are provided with tissue engaging members or connectors which can be coupled to discrete spacing links.