Rare earth lithium aluminoborate (RELAB) glasses containing dysprosium (Dy) or holmium (Ho) were investigated to determine their suitability for irradiating diseased organs in the body; i.e., brachytherapy. The chemical durability of a glass used for brachytherapy is especially important since none of the radioactive Dy or Ho should escape from the glass, but in some applications the glass should eventually degrade in the body. The chemical durability (dissolution rate) of Dy or Ho containing glass microspheres (20 to 40 microns in diameter) immersed in phosphate buffered saline (PBS) or DI water at 37°C was measured as a function of their alumina content, 5 to 20 mole percent. A highly desirable aspect of the Dy or Ho lithium aluminoborate glasses is that they degraded in PBS or DI water in a non-uniform way such that the radioisotope was confined to the remnants of the corroded microsphere. Dy microspheres injected into the stifle (knee) joint of a rat were noticeably corroded after two weeks. The in-vivo results indicate that RELAB microspheres can safely deliver localized doses of beta radiation to diseased organs and should eventually be cleared from the body.