The present invention relates to new radioactive iodine-125 miniature radiation sources (seeds) comprising a radioactive carrier matrix which consists of a porous and mechanically stable inorganic material, the pores of which contain inorganic insoluble iodide-125, and which is enclosed within a capsule made of a corrosion-resistent and body-compatible material or which is coated with such a material. The iodine seeds according to the present invention further may contain (a) usual X-ray marker(s). Another object of the present invention are methods for manufacturing said novel iodine-125 seeds.