Abstract: Radioactive therapeutic seeds include a carrier structure bearing a radioactive isotope and a radiopaque marker. According to several embodiments, the seeds include a central plug provided with an axial marker and a relatively transverse bore extending through the marker. A second marker may be positioned in the bore, thereby distinguishing a seed provided with the second marker relative to a seed not provided with the second marker. According to another embodiment, the isotope is deposited on the outer surface of a hollow radiolucent tube. A biologically-compatible, radiolucent, surface-sealing layer seals the external surface of the tube. A radiopaque marker wire of selected length is positioned in the hollow of the tube. Seeds may be distinguished from one another by providing seeds with marker wires of different lengths.