Abstract: The bio-assay were carried out by using developed colloidal yttrium-90 silicate. The results show that the physico-chemical properties of the product are stable and the particle size of the colloidal solu- tion is 170—200 nm in diameter. When colloidal yttrium-90 silicate was injected into the brain of the normal rabbites and the tumors of the miee, most of it mainly remained there and just a few deposited in the reticuloendothelial cells. This colloidal yttrium-90 silicate is nonpoisonous and no side- effect. Therefore, it can be used as therapeutical agent and the results in clinical application are satisfactory.