Radioactive therapeutic seeds include a carrier structure bearing a radioactive isotope and a radiopaque marker. According to embodiments of the invention, 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 radiographically distinguishing a seed provided with the second marker relative to a seed not provided with the second marker. According to another embodiment of the invention, 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 radiographically distinguished from one another by providing seeds with marker wires of different lengths. Each embodiment permits at least two groups of seeds to be radiographically distinguished from one another by the use of differing marker configurations. Each embodiment is further capable of being selectively marked by the physician prior to implantation of the seeds, or by the manufacturer for delivery to the physician in radiographically distinguishable sets. As a result, seeds having different levels of radiation emission can be distinguished in vivo and their effect monitored by the physician.