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. According to other embodiments, the seed includes a cylindrical outer member, and an inner member provided within the outer member and having two large diameter portions and a relatively smaller diameter portion therebetween. A marker is disposed within the inner member, and a radioactive isotope is carried on the inner member. The marker may be drilled to a smaller size. Each embodiment permits at least two groups of seeds to be distinguished from one another by the use of differing marker configurations.