Abstract: Radioactive therapeutic seeds include a two part seed capsule, a radioactive isotope, and, preferably, a radiopaque marker. According to the invention, the seed capsule has an outer part, and an inner part which fits into the outer part and which carries the radioactive isotope. The capsule is sealed by welding the end(s) of the inner part to the end(s) of the outer part. According to one embodiment, the outer part of the seed capsule is a cylinder with one open end and one closed end. The inner part of the seed capsule has a reduced diameter portion which carries the isotope and a larger diameter portion which mates with the open end of the cylinder. According to another embodiment, the outer part of the seed capsule is a cylinder with two open ends. The inner part has a reduced diameter portion which carries the isotope and two larger diameter portions which mate with the open ends of the cylinder.