An injectable formulation of the radioisotope yttrium Y 90 encapsulated in resin microspheres with potential antineoplastic activity. When injected into arterial vasculature supplying the tumor, yttrium Y 90 resin microspheres occlude tumor blood vessels and selectively deliver a cytotoxic dose of beta emitting yttrium Y 90 to the tumor site, which may result in tumor cell death and tumor regression.